True, the fact they are willing to testify in front of the senate or the FBI, then does suggest they at least believe it to be true, and so aren't going for the motivation of slowing things down.
If the process wouldn't take long (and potentially could have been done by the end of this week if I'm understanding things right, if they had requested the FBI to redo the background check last week), then yeah, seems like the GOP being unnecessarily intransigent.