Normally I wouldn't try to be an armchair game developer, but I'm leaning towards agreeing with this. the article does say:

Epic’s incredibly popular engine reportedly could not support the team's 100-player ambitions

which is a surprise to me, considering how robust it generally is. It's superficial, but I can point to Fortnite as a game using Unreal and supporting a hundred players in one lobby along with being able to spawn custom buildings on the fly.

The closest analogue I can think of is FrostGiant using Unreal for presentation, sound, and inputs, but their custom Snowplay engine for everything else in Stormgate. This makes sense to me given they're trying some tricky and somewhat novel things (at least) on the networking side that Unreal doesn't support.

All that to say: I'm very curious about the details on how Unreal was unable to achieve what they wanted at the number of players they were targeting. I'm also curious why reducing scope (either in number of players or feature set) wasn't a viable option, especially after so long in development. I don't mean this as a "they're obviously dumb and wrong", I am genuinely curious what was planned and not working. I hope we get more details because cancelled games (and the development process as a whole) is fascinating to me.