As others have said, we can safely allow positive gain in RoomFit because with "Non-Boost Mode" enabled we ensure that the total gain of all filters running together never goes above the 0dB line, like in this example:
I.e., even if some individual filters have some positive gain, when they are all working together the gain of the correction we be 0 or negative, so there's no danger of creating new resonances or ringing in the room.
In the example above you see the filter line is completely either at or below 0dB line, even though you can see that e.g. the PEQ filter #1 at 25Hz has a gain of +2.7dB.
If "Non-Boost Mode" is disabled, then positive gains in the
total response (i.e. response above the 0dB line) can indeed cause resonances and ringing elsewhere in the room. Here's one example of that, where you see that the line now goes over the 0dB line and creates a very strong peak at 200Hz, which will likely result in a resonance and ringing at least at some places in the room (
source):
View attachment 32432
In older versions of RoomFit "Non-Boost Mode" didn't yet exist, and some other room correction tools don't have an equivalent mode - which is probably where the conflicting recommendations originally came from.
Note that high-value negative gain is not so much a problem, as we're not that sensitive to notches as we are to peaks/resonances - and we anyway need significant negative gain to counteract strong room resonances at the listening position.