What makes the THR input well defined in this circuit
That's a good question. My first thoughts were that something internal was acting as a pull-up. Examination of the block diagrams doesn't reveal any pull-up.

No sign of a pull-up on the THR input here.
My guess is that the diode's leakage current may be enough.
From Table 1.1 we learn that
Threshold Current (max) = 50 nA at VS = 15V

My theory doesn't hold up when we look at the 1N4148 datasheet.

At 10 V reverse voltage the reverse current is only about 10 nA. Vishay 1N4148 datasheet.
The only way this theory could work is if the THR input is a guaranteed current source (rather than sink) in which case the output (and diode) will tend to charge up the input capacitor. The datasheet is not clear on the current direction.
Let's see if anyone else has some wisdom.