I don't think that being devastated is going to help you.
At some point, cisgender people also criticize other cisgender thinking that they trans and making fun of other cisgenders that way which is... really cheap!
They probably know nothing at all about your past, what you have been through and everything, so I can understand that you feel bad about it especially in you case being 4 years post-op.
However, I think one thing we, trans people should consider is that there will always be a way to be clocked whatsoever, whether by someone really mean or just by someone saying that "for fun.
Just move on, this does not only happen to you, or just to transgenders (maybe a bit more, is there anybody with statistics about that?) but to cisgenders people as well... never forget that human beings are incredibly capable, stupidity-wise.
And even though you have been clocked, then what? Plus you are post-op so well technically-speaking if you put aside all the gender theory stuff aside (and don't get me wrong just to put yourself in the shoes of this person who had nothing else to do) then so far you don't have a hmhm between the legs so technically speaking you are "safe".
I must sound a bit like "easier said than done" but trust me this can happen to literally anybody and the best you can do is just to move on. Basically, don't take it too seriously cause to me it really sounds like a childish conversation... and I would care much about it.
I mean if everybody was doing that around you, always, all the time. Ok then maybe in that particular case you could ask yourself: Am I doing something wrong? Actually even if that was really the case I am not so sure that you would be doing something wrong. The a majority in a group has never been a sign of intelligence neither a evidence of being right whatsoever: who brought Hitler to the power back in period prior to the WW2? Just people).
Keep in mind that there is not such a thing as being right or wrong it's all made up by people around who all have different point of convergence and divergence and all in all accordingly to their beliefs just like yours. Why bothers to overthink about it, this is simple as live for yourself and people you love and not for the peace of mind of people who do not live in the same reality as you and the ones you love cause they won't think that much usually about what they are going to say (whether it's about you or others).
I'm aging and so far, one thing I am starting to understand it's that: never take something personally.
Worst case scenario always try to understand this person or just don't bother and as I said and move on.
Do democrats try to understand republicans? No, the other-way around, neither. (I digress a bit but you get the gist)
In a life of person, there are plenty of things that does not worth your attention, and this probably includes this small tiny little thing you have been through recently