No, yeah.
> There's a pernicious narrative the United States that christianity has a monopoly on positive, moral behavior /.../ it doesn't take long to find examples of christianity creating atrocious behavior
Sure. These are not the commenter's claims, though, so it's odd that they're what you're litigating.
"We did a charitable thing as an expression of / motivated by our faith" is orthogonal to whether some portion of US Christians are either parochial in their moral philosophy or even not particularly good people.
Claiming otherwise doesn't make it look like the they're the one with a hobby horse or over-evangelical approach.
It's sure implied by that wording. I think the term that's en vogue these days is microaggression. The exact words "because only christians would do something empathetic on a first date" weren't used, but the implication is there.
But the commentor's claims seem limited to Christianity as a motivator for their behavior, which is hardly objectionable and a very different thing from your challenge-reading of their words.