the Guidelines seem to allow both <p> and <ab> in elements of the
teiHeader such as <editorialDecl>, <quotation> etc.
In a couple of gitHub issues   there seems to be a preference for
<p>. Those issues mention that in the future only <p> many be allowed in
this context, although in  (find "lb42 commented on 11 Sep 2017") Lou
Burnard disagreed on this. In any case, for the state being, both <p>
and <ab> stay legal.
Within he ALIM project  we are discussing the option of recommending
our collaborators to only use <ab> because (I quote from Lou's comment
in ): "It has always seemed to me that blocks of text in the header
are not really paragraphs in the same way as paragraphs in the body of a
text are" and to facilitate software creation.
What do you think?