I don't know why I'm getting down votes for this, its a reasonable question.

To answer my own question triethylborane is indeed used to ignite the main fuel.

https://www.quora.com/Why-was-Triethylborane-used-as-a-start...

You’re getting downvoted because you’re calling out an inconsistency that doesn’t exist. The fuel that doesn’t burn is separate from the pyrophoroc stuff used to start it.