At risk of repeating myself: determining whether damage has passed information-theoretic death falls under the purview of physics and cryobiology, among a variety of other fields. That is not at all the same thing as saying that it is "in the area of expertise" of every physicist and cryobiologist, and thus a (hypothetical) survey of them would be "expert opinion".

I agree with what you are saying here and think you've struck the right balance between acknowledging genuine competence and expecting universal expertise over general fields of knowledge. Elsewhere and at a different time it might be worth having a conversation about how to select experts in subjects similar to this one. There is something of a recursive problem in as much as it requires knowledge to know which experts are the ones that are relevant or knowledge to know which person to ask-which-people-should-be-asked. Different beliefs about how to choose authorities to believe seems to be a huge source of disagreement over a variety of problems and frequently results in "reference class tennis".

It seems that you are venturing into No true Scotsman territory:

Clearly you can divide any research field in sub-fields and sub-sub-fields. You could even try to argue that nobody is an expert in anything they haven't pubished a scientific paper on (and even then, maybe the paper had multiple authors and Author 1 was not an expert in what Author 2 did, and the referees who did the peer review weren't really experts, and so on...), but I don't think that would lead to a viable concept of expertise.

Realistically, we expect researchers within any relatively self-contained field (such as neurobiology or cryobiology) to understand the general principles and issues of their field well enough to tell potentially viable scientific and technological ideas from fringe stuff unlikely to work and outright crackpottery.

Moreover, we expect scientists to be accurate in estimating the level of their own understanding within their field (or at least err predominantly on the side of underconfidence, as per Dunning–Kruger ). Therefore, when multiple scientists independently make a claim about something within their field, barring evidence to the contrary, it seems fair to assume that they know what they are talking about.