When you cut through the corp speak in that message following stands out
  • board thinks Elon is a risk going forward ("we were ... clear about the risks")
  • board thinks Elon won't act in best interest of shareholders ("we also believed" in past tense)
  • Elon went through background check and formal acceptance - that didn't work out. One of those two things failed.
  • Twitter CEO is really unhappy about Elon ("I believe this is for the best")
    • Quite possibly the CEO is worried about hostile takeover and being fired (similarly to the hostile takeover of Instacart, where Apoorva Mehta was essentially fired by the board)