Scaling up from the base resolution is completely okay and I would actually recommend it because the resolution is very hard to play on most modern computers.
It is fine to scale. The rule is trying to expressing that you have 84x48 rows and columns of detail. The game resolution does not matter as long as you could make a grid of 84x48 and everything lines up without extra pixels poking out.
I made a reference picture of a 84x48 and 840x480 text. Both are legal the small one is just really hard to read on a 4k monitor.