Flew HIR-POM-SIN last year. Certainly did not have a visa. You can definitely do the transfer without going through immigration.

They have a security check for connecting pax and that was a bit unorganised.

I also recall they want you to go to the transfer desk with your ticket / boarding pass - although I think I was issued the POM-SIN bp already at HIR.

Having said that, the terminal at POM is perfectly fine for a few hours.