What's ambigous about being cleared to the IAF for any particular approach? That's the limit of the clearance while also giving you a prior warning about what approach will be used.
It's equivalent to recieving a taxi clearance touse xx runway. No one expects that to mean to enter the designated runway. We all know you're expected to seek a further clearance to continue onto the runway and stop at the holding point if not received.
Same with the 'cleared to' phrase.