1) Most of their advanced air force consists of American hardware. That automatically means they're behind the US in technology.
2) You can't attempt to claim that title without long-range bombers. B1? B2? B52? Anything? The tech, targeting and global deployment capabilities to support all of it? Nope, Singapore has nothing there.
3) Their drone programs are very far behind what the US had ten years ago. They're relying on weak, short-range Israel UAV tech. The IAI Scout is the core of their drone capabilities and it's 40 year old tech. It can only fly to 15,000 feet and at 109 mph. Then the IAI Searcher, which is only capable of 20,000 feet and 125 mph. Those are toy drones.
The Global Hawk goes to 60,000 feet, and 357 mph. And that's considered old US tech.
4) Their global air force integration capabilities are non-existent. That includes complicated logistics and the ability to fly anywhere globally, and refuel for long-range strikes while ensuring the safety of all planes in question.
That means the Singapore air force can be destroyed locally without ever posing a threat. That's not an advanced air force, it's a sitting duck. China would dismantle their air force in a matter of a few days.
5) They have no space capabilities at all. That includes the ability to shoot down satellites, a required capability for the next 20+ years to claim an advanced air force. They have no ICBMs, and no other long-range missile capabilities.
6) They don't have a fraction of the global spying and data gathering (targeting, situation awareness, et al) capability the US air force has.
7) They don't possess the F22, which is the best fighter jet in existence.
8) They're incapable of producing the bulk of the hardware that makes up their own air force, including the planes, UAVs, missiles, helicopters, etc. That means they're always behind and always will be by default.