I feel like it can be interpret in more than one way tbh. 
And most comments seem to interpret it similar to you. 
Usually in Pokémon, the term ‘unlimited’ is simply used for cards that are not 1st edition, so both ©1999 and ©1999-2000 are two versions of ‘unlimited Base Sets’. Then again, Shadowless would be considered unlimited as well by those rules, and usually it isn’t. 
As discussed in this thread: Better Name for Unlimited?, ‘unlimited’ has always been a weird term. Especially when you consider that most unlimited edition Japanese sets are very limited and short-printed compared to their 1st edition counterparts, haha. And they aren’t exactly still printing and releasing Base Set cards anymore in 2025. 
Greetz,
Quuador