I don't know how good the GAU-8 is, so I can't say for certain if the Merkava in the chat has a point or not. It's tier 10, so I can't really fault anyone for playing to their full ability when everyone else on the team should have a decent understanding of how to play the game unless you're constantly playing something that's T-15 levels of OP (particularly in a platoon), at which point it can vary between tryharding and "actively trying to be a dick to your teammates" (tryhard platoons are closer to "actively being a dick" territory).
IMO, I'd say you're probably fine unless the GAU-8 is even more OP than the T-15 (which, iirc, it isn't). Looking at the post-battle results, I'm tempted to say that you aren't the issue, and that the guy who called you out has the issues. Let's say that instead of having you in the GAU-8 on the team, there was a T-15. If he got those results with a T-15 on the team, I'd assume there was something wrong on his end that prevented him from doing better than he did (especially when you consider that the Leclerc did nearly as well as you did and the PL-01 did pretty well too). It might be a technical issue (i.e. potato PC, running 30 chrome tabs in the background, dial-up internet, etc...) that needs to be fixed, at which point he probably shouldn't expect to be competitive if his technical issue is that severe. It might also be an issue of skill relating to something that the player did or did not do (constantly out of position, bad reaction time and shooting dead targets, missing or bouncing most of his shots, etc...) at which point, unless he has a medical condition that prevents him from being competitive (at which point he shouldn't expect to be competitive), I'd say he should probably practice his skills and get better at the game (the classic "Git Gud"). In either case, I don't really think that he's in a position to call you out for being selfish if he is the one with an issue.
The only other thing I can think of is that the guy might be trying to get infantry damage (for example) for a mission of some kind. If this is the case, the only way he might have a point in calling you out is if he said something to the effect of "I need infantry damage for a mission" at the beginning of the match, and even then that's still iffy unless you acknowledged him and proceeded to roflstomp everything before he could get his infantry damage.
TL;DR: Unless you're driving a stupidly OP vehicle (even more so if you're in a platoon with other people in said vehicle) to actively be a dick to your teammates by killing everything before they have a chance to do anything, you're not the asshole. The only exception to this is if someone requests that you change your playstyle to allow them to do something and you agree to do so, but you either don't change your playstyle or change your playstyle to screw them over.