Jenkins's assertion has also been challenged by now ... I suppose the real interdependence between the texts will never really be resolvedJenkin's Arden edition from 1982 is quite old, though. The 3rd Arden edition (Thompson, Taylor) mentions that other scholars have more recently assumed (e.g. Erne 2003) that while Q1 may have been constructed from memory, it may be based on a version of the play that Shakespeare wrote for the stage (because Q2 is way too long to be performed)
In the 3rd Arden edition, you read for example:
"Charles Adams Kelly (...) has consistently argued that the text of Q1 could not have been derived by memorial reconstruction from either the Q2 or the F text. He is particularly struck by the absence from Q1 of ten Q2 passages which are not in F, and three F passages which are not in Q2. Together, these passages total 259 lines, and Kelly calculates the statistical certainty that these lines could not have been in existence when Q1 was created. This and other evidence has persuaded him that Q1's text predates both Q2 and F, and he has also come to believe that the Der bestrafte Brudermord text derives from an even earlier version."
"In 2014, Margrethe Jolly's The First Two Quartos of 'Hamlet': a New View of the Origins and Relationships of the Texts and Terri Bourus's Young Shakespeare's Young Hamlet: Print, Piracy and Performance each argued that they could explain Q1: Shakespeare wrote it. Furtermore, they believe there is no need to imagine that there was any other Ur-Hamlet, since Shakespeare could well have written it as early as 1589, the year that Thomas Nashe referred to 'whole Hamlets, I should say handfuls, of tragical speeches'. Jolly has little to say about F, but argues that Q2 represents Shakespeare at some point between 1598 and 1604 revising the text behind Q1. Bourus goes further: her thesis is that Shakespeare revised his play twice, the text behind Q2 being a revision of the text behind F, while the text behind F, composed in about 1602 but not published until 1623, was a revision of Q1."