BIP 91 locking in is just a promise. If the miners that signaled will keep their word and signal for BIP 141 (actual SegWit) and reject blocks that don't signal, then and only then will BIP 148 become irrelevant.
But isn't that now basicaly more or less a formality or could there be a bit of manipulations behind?
Yes, it is essentially a formality. No miners signaling for BIP91 is going to be crazy or dumb enough to not actually follow through when they'd be stepping on their own toes. Not to mention, if it was their intention to not execute then why signal to begin with when you could have just not signaled? Sometime in the next 48 hrs BIP148 will become unnecessary.