Here's the context:

"We underestimated the gravitational pull of existing platforms. Network effects aren't just a moat, they're a wall."

It's a mixed metaphor which doesn't make any sense. There are really very few ways in which this can be considered good writing - I guess the grammar is ok even if it is nonsense.

So let's break it down - underestimated the gravitational effects - ok, this is nice, like where it's going talking about these big competitors sucking in users, but then we have the metaphor extended to breaking point:

Network effects are a moat, but not just a moat, they're a wall (which is really not anything like a moat). So which of these 3 things are they, and why are we mixing the metaphors of gravity (pulling in customers), moats (competitive moat) and walls (walled gardens).

It's just all a bit nonsensical and the kind of fuzzy prose that seems superficially impressive without actually saying anything meaningful in which LLMs excel. Go try generating an article from just the heads in this article, and see how similarly it reads.