Yes, that is route they are relegated to as they can't fly over Pakistan Airspace which reaches out at Sea as well.
Since Afghanistan is landlocked, they (IAF) can either enter from the North or through Iran.
As you can see from the image (above), they fly off right at the edge of the Iran/Afghanistan/Pakistan Border.
It is somewhere between
Zahedan &
Zabol in Iran, before entering Afghanistan.
I think PDF's resident Pir,
@Imran Khan bhai may have more information as he's practically tracking the movement of each every IAF Transporter by the hour on
Flight Radar 24/7.