Well I wasn’t fooled by fake Luke not even for a second, but didn’t know how he was doing what he was doing until revealed.
What was he doing again? Was it stalling the First Order so that the 5% of the rebels who Poe hadn’t already gotten killed could sneak out the back even though Luke had no way of knowing that there was a way out the back, and in fact there wasn’t until Rey conveniently created one with the force when she just happened to show up in Falcon which was doubly fortunate since without the Falcon they’d all just be standing around out back waiting to be found an executed?
This was Luke’s plan? Buy the rebels 5 more minutes in hopes that the salvation that’s eluded them the whole movie will show up (now that most them are dead anyway), and of course it does because movie.