If the logic is transit no need check then why Taiwan still check?
Cos it seems that this Sri Lankan bought a fake Canadian passport, so he transited successfully through Singapore, but he was also transiting through Taiwan, so why Taiwan custom still check?
If there was sufficient time and he left the transit area and entered, then they could check. Or alternatively if he behaved suspiciously they might still conduct random checks.
Anyway, in this case, it didn't sound like he was transiting in sg. If he had boarded on sg, ica would have checked him. I doubt he would have gotten thru since for icao passports, they scan the chip as well, which is encrypted. My guess is he might have used original sri lanka passport during departure.