There is no rule about moving when braking, full stop. When Alonso does it, and other drivers, and quite frequently, it is called robust defending and pushing the boundaries.
In that video Max is only drifting across the track and to prove he deliberately moved you would need the telemetry from the steering wheel to show that there was actual input. Which tv couch 'experts' do not. We simply do not have any of the evidence that the stewards have and from that video Max is moving into the path of Lando but there appears to be no sudden dart, unlike what Alonso and other drivers have done in the past, and Max's car is drifting into the path of Lando's even before each car is alongside. Lando should have seen that and backed off.
However much people are trying to whip up a storm about it, because it is Max, in racing there is little to shout about, much worse has been done by Lewis, Alonso etc, and even by Lando himself in the past.
Seriously, we want racing not dictated to by rules and to suggest that because someone presents an alternative reality is defending Max is just no more than a cheap shot. It could be argued equally that some seem have a problem with Max, and Red Bull, in general, and thus make it personal and will just keep on obsessively focusing upon everything Max does, that every other driver has done, as a criticism.
Max got a penalty for causing a collision, time to move on...