TL;DR;
The board basically caved to the demands of the 500+ employees that penned and signed a letter telling them to reinstate Sam Altman and disband the current board.
In addition to reinstating him, most of the board is "quitting".
As others have replied, the MS thing is about them having a Plan B in case the board didn't reinstate him.
One way or another he's working for Microsoft here at the end of the day, Microsoft is just making sure they keep him gainfully employed within their control. Their move to hire him was covering their bases.
If the board were to reinstate him, like it has, then MS can rescind their offer and allow him to join the Open AI CEO position again.
Either way, MS didn't want to lose Sam is all this says.