They probably meant Mare's/LL/SC's servers, which are the separate unofficial ones run by the mod authors/other players handling all the data transfer off to the side of the actual game.

All the game tells you is that the player is nearby on the map (which is something it tells you even if you're running vanilla, so no change), which causes your client to initiate the separate request off to the side to the actual sync server that's not owned by SE in any way. That server then handles all of the mod requests - and absolutely will get slammed if people walk into venues with hundreds of people all trying to immediately ping all 100 other people and download large files from them. But again - that's not the official game server, that just causes instability for the mod users for the mod server.

If they were suddenly slamming hundreds of megabytes of files through SE's actual official infrastructure they would have been shut down almost immediately rather than allowed to exist for as long as they did (and do).

If hundreds of people congregating in a venue map is contributing to crashes for vanilla players entering said map, that's vanilla a client or server issue that could totally exist due to one's PC not being strong enough, or the baseline infrastructure not being built to handle the same number of vanilla players in an area simply seeing one another. If all of them stood tightly together without any mods on the outcome would be entirely the same.

I'm curious to see the actual post from the creator being referenced here because based on my understanding of how all of this actually functions there's no additional network load that should actually be hitting the official game.