More teams working on features in parallel leads to all kinds of fun synchronising the changes, getting the code bases merged and integration tested. More managers leading the teams and producing Gantt charts. More time spent in meetings, more everything around a limited amount of additional effective code changes.
The developers also get held up, being told they can’t release A until the work on B has reached a certain milestone.
You will certainly gain speed adding teams and developers. They are writing code, after all. But the cost will in no way be commensurate to the perception of increased pace from the outside.
For all we know, that’s exactly what Epic had been doing internally.