Adding manpower to a project adds overhead. In a late project, the benefits will happen too late hence Brook's Law.
In this case, people are anxious NOW. They want updates NOW. Adding manpower now will probably delay things that would happen shortly, but improve things that would happen later.
So adding manpower is another long term approach that will not immediately solve the problems some people are complaining about, but make them worse. Right now they have made it clear they are working on the engine, which is something that costs now but will pay back dividends later in development. Lots of people aren't happy with that either.
People want updates NOW. If you want them asap then don't add developers now. If you want to look at the project as a whole and sacrifice some delay now for improvement later, then support both the engine development and hiring on new staff and be more patient in the short term.
Brooks's law applies.
Brooks's law doesn't apply because BY DEFINITION Brooks's law ONLY applies to projects that are late.
Yes, it might slow it down current progress for a bit (would we notice?), but not nearly as much as adding people to a project that is almost finished because there is less to learn and more opportunity to innovate. The new guy they recently added seems to have picked things up quite quickly.
Also, I don't think any of the reasonable folks would be real upset if facepunch said "thing are going to be slow for the next few weeks as we get new devs up to speed, but after that progress will be much faster than before." The unreasonable folks might, but the unreasonable folks will complain about everything.