Is it libel if the criticisms are true and demonstrable?

Of course not. If it's true it can't possibly be libel.

Libel must be a statement which, by its very nature, is an allegation of fact. It must be untrue and known by its issuer to be untrue. Honest errors are not libel. Further, it must injure the character or reputation of the victim. People like Jack White who seek public exposure actually must prove a greater degree of injury from libel; Jack White chooses to speak and write on controversial topics and thus he is expected to deal with criticism.

Jack White threatened to sue Clavius for libel because we claimed he was not properly trained in photographic interpretation, certain sciences, and photogrammetry. His claim would not be legally defensible because I have (a) statements White made under oath admitting the substance of Clavius' statements, and (b) recent statements issued by White indicating his lack of such training.

Those who deal with White on a regular basis assure me that this is typical Whitian bluster which is simply his method of dealing with opposition to his findings. White, according to them, is simply an intellectual bully.