When the Commons Speaker & the EP President meet, that's a completely standard meeting of counterparts. It'd be a diplomatic snub for Sassoli to come to London & not meet Bercow, & likewise if Bercow refused to meet Sassoli.

Obviously, they weren't gonna pretend that Brexit isn't a thing when they met & spend the time talking about the weather instead. But there's no indication that they met to "prevent a clean brexit" in any significant way (what could Bercow even say?); the conclusions in the press statement about the meeting are as bland as they could be.

Not to mention, Sassoli literally met Johnson the day before he met Bercow. Some "behind the backs of the UK representatives" that is.