Have you maybe noticed that pack sizes grow? I remember times when you were called crazy if you had 50 mods installed. Anything like 20-30 mods being installed was called huge.
Nowdays you see modpacks that use 200 mods overall.
But the memory usage comes not from minecraft. Minecraft 1.13 uses 300-600MB of ram around the same that 1.12 uses.
The memory increase comes from LoadedTextures, and loaded Models, audio files, and barely from any systems within games.
So If we speak about “Adding More ram” needed to make 1.13 run. I will say yes this is FAKE NEWS.
As of this moment. That may change if mods start to import massive amounts of memory.
Minecraft itself will not cause more memory to be used. Actually they improved Systems to reduce Active Memory Usage by a lot.
Structures are no longer loaded permantly in memory, they load in as chunks load. (Yes thats a big memory usage)
Villages will be removed with 1.14, so the village bug will be fixed with it.
Chunks have now proper heightmaps that are improved in memory usage.
And a couple more.
So what serverowners suggest: Hey lets double memory usage because 1.13 is going to use more memory is clearly a fake news in the way it is explained.
The real news should be that ModDevs have more interactions, code quality is dropping, memory leaks/data pools are being used a lot more then before so we suggest if you play with a lot of mods you should increase your Ram just to be sure.
So this has nothing to do with “1.13”, if 1.12 exist long enough (Like 1.7.10 did) then memory usage on mods grow on its own because features get added.
Why do host suggest more RAM for users. Because the biggest amount of players just take a finished modpack and run with it. And modpacks go high in modcount, have a lot of active scripts and maybe also a lot of custom assets outside of what mods install.
So they want to be save and suggest how much ram should be used so you can be save.
It is the same reason my tool says you need at least 3-5GB of Ram on top of what the game takes to run the pregenerator because people will use most likely the biggest setting which caches a lot of data while the pregeneration takes around 1.5-2GB while it is running (without the cache).
Remember most of the users are plug and play users, they do not go into the matter, and they are not interested to go into it.
Why do you think a lot of big game server hosts have dedicated developers.
So TL:DR: Yes what they say is fake news because the growth in ram is not Minecrafts fault but the Mod Creators/Forges fault.