I can't reproduce that.

I can reproduce this. It might be only happening for Russian IPs because it does not happen if I use my VPN.

(The author is Ukrainian, so nothing surprises me at this point)

As per the OP, there's no advantage of doing this if you're in a region that Tesla officially supports, because Tesla already has free connectivity with the free built-in SIM. The only use of this is if you're in a region where Tesla isn't supported, and thus Tesla won't have the necessary roaming agreements for its built-in SIM to work. The Calyx sim only supports US and Puerto Rico, so it's of no use. There's no privacy benefit either, because the phone can still upload your location/telemetry regardless of what SIM you're using.

Does this mean spam sms could be waking up my car even without a sim swap?

Does it mean that if internal DB of internal numbers will be leaked anytime in the future, it will give to anyone power to wake up all Tesla cars in a few clicks?

I think about this every time my car is slow to wake or doesn’t wake when in a spot with poor signal, it’s SMS fuckery.

Regardless of all their problems now, decisions like this that made Tesla successful. Need a low power always on way to send the car a signal to wake from a mobile device? Don’t invent and build some complicated hardware and protocol, just use SMS and a small cell phone board.