That is correct: without an external throttle return spring the throttle plate stands 1/2 of the way open. This was done on many Carter carbs (and some Strombergs) so that the accelerator pump was constantly "pre-loaded" no matter how much wear might be in the linkage.

I experienced the downside of that engineering for myself in 1977 when the throttle return spring broke on my '67 Plymouth and the engine immediately accelerated to half-throttle. Luckily, I knew exactly what to do (thanks to good prior instruction) and an accident was avoided: pop it into neutral, shut off the engine and coast to the side of the road.