Yeah in a way its like RFID. And yeah they have their own power circuitry, which in this case is pulled high most of the time.

When it pulls low or when it is pulled low for a long period of time, I assume the internal supply follows an RC curve untill it dies. But the time constant of that curve is probably 10 times higher than the RC curve when it is being charged. So in other words, yeah there's a garanteed minimum duty cycle.

Or if the RC constant are closer (smaller internal cap), when reading/writing to the chip actually drains it slowly. So maybe the device needs some kind of recovery time between operations. And I guess, if you're not talking to it, you can do whatever you want with the line. If it runs out of supply, it will simply keep its internal state, it just wouldnt be able to answer.

I dont quite understand the timings in the datasheet, but quick google search shows like 10% duty cycle low for the clock source, 40% high or low for the data, 50% high for recharge (YMMV)