One thing to understand about software development is that adding people on projects doesn't have the sort of linear increase you might expect.
If you've got a deadline in a month and have 2 months of work with your current staff, you can't just double up to get there. There's a lot of missing knowledge your new devs don't have, and they won't be able to just leap in and start running immediately. In fact, in the short term, adding new employees tends to cause productivity losses for your current employees, because the new hires will take up time from the veterans in order to get up to speed.
This is before we get into things like team structure, process structure, can your current setup accommodate more people or do you need to change up in order to manage a team of that size, etc.
On a long enough timeline, adding more bodies works and is necessary, but you have to be careful when doing that.
That makes total sense actually and basically mirrors my own profession. The reason I made doubly sure to put emphasis on being an outsider looking in, one always misses the finer points without first-hand knowledge.