You're the one being disingenuous by focusing on one transfer window and attempting to find the manager equally culpable for a recruitment failure that pre-dates his arrival by years. Pep may not have felt defensive signings were a top priority before he started working with the squad but I bet he sure as hell does now, and that's the whole point of having a DoF in place, so that you're not constantly having to rebuild the squad every time a new manager comes in.
Guardiola knew all about our squad before he arrived. Not only had he done extensive research on us before the 4 games we played against his Bayern side, but he'd been to see other games like our matches vs. Barcelona.
After that he had his backroom staff create dossiers on every single player at the club for him to go through before he arrived, and he was with most of them for 3 months before the window closed.
So he had loads of time to assess the quality and capabilities of the fullbacks and chose to reinforce other areas instead.