Having separate tickets doesn't mean you cannot use air-side transit.
Check-in baggage or not, it also doesn't mean you must go land-side.
I have certainly done separate tickets, with check-in baggage, and used the air-side transit without having to go through customs/immigration or claiming my bags. Bags retrieved on my behalf and redirected to my final international destination.
Done it multiple times. Whether this is possible depends on the airlines involved.