You should all go check it out right now.
Anyways I've been thinking on this for about a day now and I kinda think... well the thing is.
I don't think they're quite on the same page regarding what their fight is actually about.
The meta linked above is DEFINITELY 100% the argument Hardison is having. Textbook, to the letter, that is what he's arguing about. OP spelled it out so perfectly, that I have very little to add here. Parker having to deal with the rift that causes and the SPECTACULAR analysis of expressing herself through the cards.
But yeah Hardison's whole thing of seeing Eliot The Sandwich being free game now that it's in the fridge. Up for grabs. Him stealing it. Etc.
But that is NOT the argument Eliot is having.
A Communal Fridge is just Not A Thing for him (yet). He's not used to the very idea of it. Hell, even just partnering with other people on this level is still fairly new to him, especially in the light of how long he's been working alone. The very concept of a Communal Fridge escapes him. He doesn't even realize that sharing is on the table.
He's used to, well, "a man makes [read: sets aside for himself] a sandwich and expects it to be there [read: still his, that boundary respected by everyone who knows he's set aside that andwich] later" and "you don't steal another man's sandwich!"
But you see, the problem isn't Hardison stealing the sandwich, it's Eliot seeing his own desire to steal the sandwich reflected in Hardison's actions. He's not arguing with Hardison, he's arguing with himself AT Hardison.
Because he wants to steal the sandwich. He wants to steal Parker The Sandwich from Hardison on one hand, and Hardison The Sandwich from Parker on the other (still seperate in his mind for now - TWO unique transgressions, not just the one) but they're his friends and you don't steal another man's sandwich - in fact, you personally make sure they have enough to eat - so he's trying his best to pretend he's not hungry, not starving... and then Hardison has the gall to go and-
And then "nobody throws Hardison off a roof... except maybe me, haha... no."
Not gonna let anyone hurt him, and that would include stealing Parker The Sandwich... except maybe, if it were me, since we're also so close and kinda also... No. No no no. Bad idea.
Anyways Everyone Is The Sandwich and polyamory is the Communal Fridge that Eliot didn't even know was plugged in yet.