Ether Jones wrote:

If the igniter doesn't light, the gas valve will not open at all

That sure sounds like the safe way for it to work.

And it seems to agree with what I observed: the igniter was not
glowing, and I couldn't smell any gas at all.

I do wonder something though. IF the gas were to turn on (but not
ignite), would the smell be detectable? Or is the vent motor so
effective that all the unburnt gas would be vented and you wouldn't
smell it. Just wondering if anyone has any experience with such a
situation.

It isn't necessarily the correct info. The replacement for your obsolete
control uses timed sequences only. As a supposedly improved version this
seems to imply that the original wasn't safer than the replacement, or
IOW, the old control likely uses timed sequences as well. The gas valve
is opened 17 seconds after the HSI is energized.

BTW, the flame is detected via flame rectification, in exactly the same
way that units with separate flame probes detect the flame. The position
of the HSI in the flame is thus very important. 1/2 to 5/8 of its
surface should be in the flame.

Richard Perry