That is true-- the first time DD had any egg-containing food (and it was traces in a mix), she had hives all over her body within a few minutes after just a couple of bites. Less than a month after she anaphylaxed in spectacular fashion from her first known exposure to peanuts (~100-150 mg of pb).
So I think that most of the time, it just isn't possible to identify with a great deal of precision exactly what a sensitizing exposure is. Barring special circumstances, of course. We're pretty sure that we know what sensitized DD to pistachios, for example, since she's very obviously never eaten them (even in trace amounts) in anything, given her PA diagnosis so young. We think that one was from a shell that she found on our family room floor and popped into her mouth when she was about eight months old.