Only because the AF got really careless and literally overflew the radar site repeatedly on the same path at a predictable time.

Stealth is a not a cloaking device. It's a <1 multiplier on enemy radar range. If you get close enough, you will still be targeted. But combined with sufficient standoff range on your weapons, stealth enables hitting your target before they can hit you.

The serbians were only able to shoot the F-117 down because it had its bomb bay doors open. These days the bomb bay is automated. Meaning it will only be open for the 1-2 seconds gravity needs to get the weapon out of the bay.

They were able to shoot down _one_. This was after analysing many sorties and a bit of luck under ideal conditions. Radar has improved but so has stealth and operating procedures.

S-500 only went into production this year and with sanctions in place not sure how many they will be able to build (if any) with their supply chain in tatters.

And given that Israel has been running around Syria with their F-35s being undetected by the S-400 not sure how much confidence anyone should have in Russian claims.

Have you considered that maybe just maybe Russia is not keen on starting a kinetic war with Israel? Not shooting down doesn't mean not detected.

Also, Russia has already delivered some S-500s into service. Their supply chains might be strained, but they are still functioning & mostly domestic sourced.

That doesn't make any sense.

You don't deploy billions of dollars worth of infrastructure in Syria just to leave it switched off.