I think the jury is still out. You present a good case, but it
depends on quadrupedality in all pterosaurs, something that has not
yet been documented. This is pertinent because they came from
bipedal ancestors.
I bet that if birds _could_ walk or at least stand quadrupedally, they'd
launch quadrupedally, too, by means of the mostly forelimb-driven jump seen
in vampire bats and reconstructed for pterosaurs. Why bother carrying strong
hindlimbs around that are almost never needed, when you can use the already
much stronger wings for the launch?