The argument against counting them in the total is that foil cards aren’t really design.
If you’re taking a “common sense” approach for what counts and doesn’t count as a unique MTG card - namely, that if you look at two cards side by side is there a clear and intended difference between them and way to tell them apart - then clearly foil card variants count as unique, different cards. So there’s some debate over whether foil variants should count as a unique printing in the same way new artwork or a new expansion symbol more definitively count. The foil is the same art, same expansion symbol, same card number, and so-on.
Unlike the differences between the 8th and 9th Edition Remove Souls, the foil cards are the exact same print as its non-foil version. What this means in practice for us is that, since 1999, almost every new Magic card print also exists in a much more rare shiny foil version. I’ll look at this in more depth in a future letter. The odds have varied and there are exceptions (don’t me) but very roughly about 1 in 45 to 1 in 100 total cards distributed in Magic booster packs will be a foil replacing a regular card.
If you don’t know, beginning in 1999 you had a small chance of any regular card in your MTG booster pack being replaced with a shiny “foil” version of that card instead. Foil cards have their detractors, me included (I think they’re tacky and frequently make card art look worse).