Well, immediately after that, they say they inverted the entire curve (through a naïve 1/|X| and then un-FFT, presumably) and saved it as an impulse response. I think the 300Hz slope is to _keep_ the rolloff in that area, so that the system doesn't try to boost bass by 40 dB and just end up clipping madly.

Since these operations sort-of commute, it is easier to understand if you invert the order of operations:

1. Invert the spectrum (ideally through something less sensitive to noise than what's done here). Now you have an impulse response that will, in theory, give you a perfectly flat frequency response.

2. Apply a 300 Hz slope. Now you gave up some of that flatness, but you have something that's physically realizable without murdering your speakers.

Exactly correct, that was my thought process while doing it. I tried it without that 300 Hz slope first of course, but that did try to murder my speakers. :)

Now I understand, makes sense!