I wouldn't jump to conclusions just yet about how Johnson will react to this, especially after he backed off with Cummings.
It was widely reported last night that Johnson was totally pisst off about the Cummings issue and how Dom had dropped him in it. Apparently he used to remind him about regularly. Johnson might just see this as an opportunity to lay the law down. After all this is not going away. There is still the issue of the Home Office's top Civil Servant's industrial tribunal to go yet where all this will come up again so Johnson may well decide to sort the issue now. Especially as she is proving to be a complete disaster as Home Secretary and he will have to move her soon anyway.