I'm not convinced Txiki is the entire reason Guardiola is at the club. A small factor, perhaps, but Pep is about more than just joining one of his mates.
And if Txiki was so influential in this, why has he been signing players Pep has immediately wanted shipped out? Mangala, Bony, etc
Without Begiristain I genuinely don't think we'd have Guardiola.
As for the signings you mention, I'm sure Begiristain didn't envisage them turning out so badly, and they came well before Pep arrived.
Begiristain has to take a large share of the blame for those signings, but it's hardly evidence that he wasn't instrumental in getting Guardiola.