I'm sorry, but I don't believe that's correct.

A few years ago I was transferring from a SQ flight from SIN to an Air India flight to DEL (see post 1).

It was made quite clear to me that the UL transfer desk did not issue boarding passes for AI, and I had to enter Sri Lanka and re check in with Air India. Luckily I did not have checked bags. And I know that there are other airlines that UL won't handle, including all LCC's.

So you may well be able to get your SQ boarding pass at the transfer desk, but it would be worth checking beforehand.