(Note that two people can be wrong at the same time. When I agree that the B responders may indeed be motivated by confirmation bias that doesn't rule out the possibility that the A responder was also motivated by (an opposite) confirmation bias based on quite different previous observations. So, the parent may be interested in a genuine possibility due to a bias and pointing to a real but not the most likely possibility (it's hard to judge how committed he - the parent concerned about delayed reactions - is, so we are bound to interpret his comments in the way that makes the most sense and assume he is making only the rational point that is there to be made); and yet at the same time those who assume he's speaking due to confirmation do so because they don't recognize how common such delayed reactions are and assume he must be stretching a point or imagining such long delays because he's desperate to preserve a prior belief. Note this study was small and quick - 37 people and 9 days, apparently.)

As of now, I don't think you can get away from confirmation bias on the part of the B responders, at least, though. If the parent is true (and yes, those delayed reactions do exist), then those who accuse that poster of confirmation bias can only get there by assuming (consciously or unconsciously) that the reactions they are most familiar with (intolerance and IgE) are exhaustive, since they can't read his mind and know that he would reject future results from better experiments that are consistent with this experiment. He (the parent) raised a real possibility therefore the knock-down proof fails, as for now. We don't know that non-celiac gluten-specific reactions don't exist. That assumption that reactions to gluten or anything else have to be similar to IgE reactions or intolerance is confirmation bias (of the common "what I've seen so far is what there is" kind) - but of course our brains are associative, and we like to save thought and energy, so there's no massive shame in that; confirmation bias is a part of everybody's daily experience in at least small ways because that's how our brains work and must work. Overlooking even the possibility of disconfirming evidence (that in fact exists in similar contexts, such as oats) is quite a strong form of confirmation bias.

It's worth noting that the study doesn't show that there aren't reactions to gluten that aren't celiac reactions; in fact it shows that there are people who have such reactions, it's just that they also react to a lot of foods (this is actually common with MCAS - Mast Cell Activation Syndrome, for example - some people with MCAS end up restricted to a handful of foods.) Such patients turn out to have a helluva lot more problems than just gluten, so they benefit from avoiding a helluva lot more than gluten.

You may to have to disambiguate "concern" "situation" and "he" if this reply doesn't seem suitable to you. But if you meant the original scientist, he didn't allege confirmation bias, even by implication so far as I can tell.