Yes, but we are discussing what happens when that gas hits the endless/boundless absolute vacuum of “space”.
Highlight “endless” or “boundless” or make it “infinite”.
What happens in this hypothetical? Those gases are dispersed in a boundless absolute vacuum faster than any forward movement or velocity could occur, plus you are forgetting the problem of building a structure that can withstand the pressure of this boundless infinite and absolute vacuum.
A thrust chamber is not “boundless”; it has a finite volume, a fixed inlet area, and a fixed outlet area. When fuel and oxygen are injected into the chamber, the flow rate in is greater than the flow rate out, as you have agreed that the flow rate out of a chamber is limited.
So, just as the water level in a leaky bucket can rise if the flow rate in is greater than the flow rate out, the pressure in the chamber rises. Then, when ignition occurs, it occurs not in zero pressure.