I've just been through a
missed connection that was covered by QF, and after talking about the experience with one particular FTer I'm starting to understand what's covered, what's not more than before.
I'm not sure whether there's any Singapore local equivalent to EU 264/2004 or FAA Rule 210, but if not, then it's whatever your CoC says. CoC has to be filed with your local CAA so sometimes they incorporate local laws and regulations (FAA Rule 210 only exist in previously-filed US carrier CoCs - the rule itself no longer exists).
I found my Qantas rights (and what is not included) in Qantas's CoC. Qantas also has a Customer's Charter but nothing in addition there.
If the Singapore CAA lets Scoot get away with such liability-free CoC then it's legal.
Another example - when trying to get to CTS from HKG via KIX with Peach, Peach insisted I buy two tickets - one international and one domestic,
and expressly disclaimed all liablity for connections.