The dealer is simply taking the position that since the vehicle was
involved in an accident (which they presumably don't know the details of),
they need to do significant diagnosis to find the problem, and won't be
sure what caused the problem until they find it.
This doesn't mean they will charge you. It simply means they're asking
you to promise to cover their time spent on diagnosis in the event they
find the problem and it isn't warrantable. It sounds like they have some
idea what direction they're headed-- i.e., they know what type of problem
they have (e.g. no power to crank sensor) but still need to find out why
that is occurring, and it's not going to be very simple.
If they do tell you it's accident related, be prepared to submit any
needed repairs to an appropriate insurance company. If the insurance
company and dealer disagree, they'll likely work it out between them. If
there's no insurance involved, they should be able to explain to or show
you why the problem is related to the accident.