I think you actually hit the nail on the head with the blu-ray comment. Phil buys tons of food that looks good, but instead of cooking it, he orders Door Dash or him and Khet have some potato based meal. Phil buys tons of blu-rays, including full TV series of arguably pretty good shows, but instead of watching them, he'll watch some wrestling event instead. Even when he actually does something with Khet and watches something with her, it's a YouTuber or something on Netflix, nothing from his collection. His video game collection is similar too, instead of playing/replaying any of the numerous games he owns, maybe even playing some of the ones with multiplayer options with Khet, he keeps pumping thousands into Champions.
I'd also point to his computer. Phil bought an expensive computer. He'll flex about how he used to host LANs and build his own PC's, but when it comes time to stream, he's playing on console.
I think Phil does try and flex about the food he buys, it's just harder for him because he really has no cooking knowledge. With a PC, he at least knows some common terms, with his blu-ray collection it's even easier, he just has to title drop the names of series/movies other people have said are good without actually describing the plot or anything. Phil also spending an exorbitant amount of money on something is a flex of its own. He spends $200+ on groceries every week because he's a mature adult, and, in his mind, mature adults buy expensive things, whether it's food, games, electronics, statues etc.
I don't think it's a case of Phil buying a bunch of cheap food. I think it's a case of Phil buying pricier items on impulse *and* buying cheap food. When he isn't door dashing or eating out at an overpriced restaurant, he or Khet will cook, aka microwave, the cheap food. To give them some credit, maybe they even try to do something with the more expensive items they purchase but it still comes out looking like garbage because they're both bad cooks. Then the rest just get tossed, just like how Phil threw away all that beef(?) in that one Cooking with the King video. But I will agree that it is possible they're buying other stuff, since we know his chain has more than just groceries. I just don't think he'd randomly decide that, instead of just using PayPal to immediately buy pulls, he instead decides on his one day off to go through the extra process of buying a gift card to buy his pulls instead.