They are fun cards to own. But is it accurate to characterize them as “very rare”? Shogakuan’s Yochien magazine has one of the largest circulations among magazines of its kind. Tens of thousands of these cards would have been produced.
I guess they might be rare if very few survived. But given how many other Pokemon-related magazine inserts from 15 to 25 years ago are still around (and dirt cheap), I doubt these cards are an exception