Seems the Qt people are ok with using it.
David Schulz does a lot of work there, e.g. he implements the language server protocol stuff, too.
Therefore I think it is OK if he is willing to do it.
Mail on creator list:
After a first look it seems that the transition to the KSyntaxHighlighter should be pretty straight forward, just some minor adjustments to the state handling. So let's wait for any kind of licensing decision and we are good to go.