I think it's more about not wanting to be mocked. It's not like Phil is required to show all parties present in his Thanksgiving dinner to take a picture. When he posted food in the past, it was usually just the food.\
That's because either Katherine doesn't want to be posted, or isn't there to begin with. Either way, she's not the type to object to what Phil does. Linda probably is, or even if she isn't, Phil can't really flex using his actual mother, so he doesn't have incentive there either. Though I'm pretty sure that regardless of anything, Phil's behavior around Linda changes dramatically so he can hide what kind of piece of shit he really is.
Even to someone as deluded as Phil, I don't think having your mom around for Thanksgiving is bad. I mean, it's Thanksgiving, I'd wager the majority of people celebrate it with their family. Phil sucks at lying, but I think even he could figure out how easy it is to say his mom visited for Thanksgiving.
Phil has demonstrated a lot of contempt for the truth being known to the public, even in instances where it didn't make any sense. You'll notice he doesn't use any of his actual pain for pitybux, those tips are only to be raked in with bullshit. We can tell because of the massive difference in how he is when he's faking shit, and when he, say, got de-partnered, or banned, during the SH saga. The latter especially was a prime example of what Phil looks like when he's not pretending to be upset over something.
Though in this case, I'm willing to go so far as to suggest that Linda was not just over for Thanksgiving, that she has actually moved for awhile. That would explain a lot more, and it makes more sense for Phil to be afraid of anyone knowing, especially if she has straight-up replaced Katherine. He would never hear the end of that, not even from the paypigs.
Phil will sometimes continue with things even when he's mocked, but usually because there's money attached to it, like the vests and hats. If Instagram suddenly paid Phil to post his meals, we'd see every single Door Dash order and potato themed meal cooked by Khet.
You're not wrong, but he has also shown that certain things are important enough to him to trudge through the mockery, and preteneding to live the robust flavorful life of a mature adult with a wife is at the top of that list, clearly. I don't think Phil flexes with pathetic Thanksgiving meal pics just simply out of his general desire to over-share, I'm pretty sure it's to say "Look at how full of meaning my life is", just like when he pretends his marriage isn't a sham. I think it's every bit as non-negotiable with him as engaging in a sham marriage to begin.
After all, Phil ditched DSP Tries It and threw away money not from the mockery, but from the work involved. It wasn't lazy enough for him, and didn't help him sell his mature adult LARPing. The QVC turkey flexes aren't so disposable to the pigroach. Something else stopped him.
