I believe that petitions for putting a candidate's name on a ballot may require "real" (i.e., ink-on-paper) signatures. That's not the type of petition under discussion here. I do not believe that a petition endorsing a congressperson's proposal would be subject to the same laws.

In any event, regulation of the form of petitions is NOT federal, but varies from state to state. So, to claim that "online petitions are not valid in the US" can't be absolutely accurate. Not the first time someone makes such a claim, and certainly not "once again." (Unless, of course, one were actually able to make the claim that such petitions are specifically ruled invalid in each of the fifty states.)

Anyhoo, this kind of petition does not have the same kind of strict, formal legal standing as a petition for adding a name (or a proposal) to the official ballot. I think that this kind of petition merely demonstrates how widely popular a given position may or may not be.

The only votes that count in an impeachment proceeding are those of the congresspersons. Citizens do not participate directly. Each representative, however, will want to go along with the general will of his/her constituents, at least in circumstances where that public will is overwhelming and undeniable. Hence the value of the unofficial "popularity contest" petition.