I can go both ways on this. I think this may constitute an illegal lottery if people are buying the packs in hopes of getting the chase card and not the normal cards that come in the packs, but I think this will be an uphill battle because the moment you declare this illegal lottery, you've essentially declared any pack of any TCG that has chase cards in it an illegal lottery. Magic: The Gathering, Pokemon, Lorcana, and others all have packs where you are guaranteed to get a certain number of rare cards and have a chance of getting much rarer cards in the packs.

I think the difference you're going to run into here, and the argument Topps may make, is that the cards themselves have no physical value. They only have value because other (non-Topps) entities place value on them. Going back to your oatmeal comparison, Topps is going to make the argument that they're selling packs of Maple Brown Sugar oatmeal that have a chance of having a few Apple Cinnamon instead. The flavors have the same value to Topps despite one being rarer, it is only the secondary market that gives Apple Cinnamon more valuable.