Man, using the word "fooled" is not derogatory in any way, nor does it imply that there is something wrong with the transgendered person being talked about. It is simply saying "Man, I thought that that person was biologically one sex, but they turned out to be the other!" That's literally all it's saying.
If it's unreasonable to generally go about daily life with the assumption of commonly established sex/gender identities, with the understanding that they may not apply to 100% of people, then get me off this ship cause I don't know what to think anymore. If I see someone who totally looks like a woman to me, but is actually a man, it's safe to say I am being "fooled;" not to say that the person was trying to fool me, had any malicious intent, or was doing anything wrong whatsoever; that's just simply the fact of what happened in that scenario.
As an addendum, I do understand that transgendered people tend to take a lot of shit, get harassed, even get beat up both verbally and physically, so it is generally a good rule to be extra careful with word choice. However, taking everything at face value, I don't think anything really offensive has been said here.