Good call, you’re correct, those disco sheets are legit.
Correct again 
Those disco sheets are not from before Pokemon was released to the public.
The date for printing the back side, is printed right on the sheet, and it’s late January 1999.
The front side was printed at UV Color, probably in February 1999.
Mistakes like that have happened, but it’s not what’s going on here.
Here, I think this will help.
Disco Foil Sheets
WotC Typesetter Chris Nitz, made all of the Protostoise. He also made all of Shadowless & 1st Edition. He created Unlimited, but it was in print for the rest of the year. He started passing the torch to the next Pokemon typesetter around Jungle. He said it was when Jungle came out, which might mean after he’d completed Jungle, it’s not clear. Chris would have only done the English versions, the next typesetter handled the international expansion.
Info from the typesetter: Either we knew something worked, or we didn’t know. This was a test for some of the things we didn’t know. A few different holofoil patterns were tried out. The team was looking for cost savings by using different holofoils, but they had to see how it would affect the final print. The gold rectangle frame around the flavor text at the bottom of the card, is overprinted on the test print. That’s why the gold doesn’t “pop”, it was adding color over color. Normal cards knockout the color under that gold rectangle frame, so that it prints on white. The corner of every card, has a tiny quarter circle. Where the corners of 4 cards meet, this forms a complete circle. That circle indicates where the sheet should be slit into cards. The color separation films this was made from, were laid out to fit any press, any sheet, anywhere. That’s also why it has a wider yellow border on the right hand side. This was made from my film, for sure something that was tweaked and rushed through just for the test prints. If it had been done with “templates”, the border would be equal on each side. We were convinced the black Length & Weight under the artwork would overprint the gold color and look good, so it was left off as unnecessary for this test. In the early days, the energy symbols and Pokémon image in the evolution box, were manually inserted by the typesetter, one at a time. These test prints were ASAP, so those things are missing. I automated the typesetting process. If a field in the database was blank, it would be blank on the card. The editors at the time, felt they could continue fiddling with the card content, and add stuff later during typesetting. With the automation, I could typeset a whole card set in under a minute per card. You type it in, I’ll typeset it out. There was no longer an excuse or reason for bad data to get into production. I was given no explanation for WHEN things would appear on cards. I separated the graphic components of the cards, so things could be turned “on or off”. I worked to have a logical way for the data to be separated for the energy symbols. Later, “libraries” were incorporated for consistency across all the different languages.
The card backs are printed in advance, because they’re the same for all sheets. The print facility calls this the “common” side, because they’re all the same. The back side of your sheet was printed January 21st, 1999 by D. Davis at PBM Graphics in North Carolina, along with many other card backs. Every so often, the press gets shut down for an inspection, and it was common practice for the person responsible for the inspection, to scratch their initials in the plate. This proves that they shut the press down, because they can’t scratch the plate while the press is running.
The holofoil was laminated on to the front side, and the front side of the sheet was printed later at the UV Color print facility, probably in February. UV capable printing presses were required to print on holofoil.
WotC would provide color separation films, which the print facility would use to burn the printing plates before production. The blank space at the bottom was intentionally created, probably to showcase the foil pattern for ease of comparison.
The bottom section of each plate was masked while the plates were burned, so that area would remain blank. At the same time the color separation film was applied to the plate, and the color bars were added. Every print facility added their own color bars, that would function with the particular quality control equipment they used. A computer reads the color bars for print consistency as the sheets pass by.
The Disco Foil sheets have the same layout as Shadowless and shadowed Unlimited cards, and was printed near the end of the Shadowless time frame. Cutting would have required shipping the sheets back to PBM Graphics, but no factory cut Disco Foil cards are known to exist. Instead, the whole sheets were sent to WotC.
I think you’d enjoy seeing my comment on 1st Edition stamp variations.
… and probably also my comment on the Pikachu variations, because it’s a good example of putting the various print runs in order.
Somewhere around here, I also posted info on why shadowless changed to shadowed, but I don’t recall which post it was on, so I can’t link it.