Go play the campaign in the 2011 version. You will see what he means.
He is exaggerating by saying AI doesnt’ send ANY attacks, they do send, but they send only half as many units, if that much, and rarely send heroes.
The only campaign mission which seems to have no issue with unit amounts or heroes (I think) are the only 3 maps they actually reforged, as in Dalaran, Silvermoon, and the Culling. The rest send almost no troops. And for various (mission-structure related) reasons, all 3 maps are way easier in Reforged. In the Culling, you only have to defend one side, BUT Mal’Ganis now for some reason uses Infernal, making it much harder to kill him (even trade all in all IMO as long as you don’t ignore MG). in Silvermoon it’s a joke because Silvermoon has almost no construction buildings, attacking Silvermoon is like attacking a 90/90 army. You also do not care if the runner gets through, because the waves they send at you are tiny. A few soldiers in a zeppelin and that’s it, except once where they sent an archmage with a few balistas and archers as they’re supposed to, I think it was the first attack.
It’s the most obvious in Twilight of the Gods. AI is supposed to send a hero every wave after a certain point, and then start sending two heroes at a time. The AI, from start to finish, only sends one hero every other wave, waves have HALF as many troops (documented, someone played TotG in an old version of the game and the reforged version, he had half as many unit kills and 1/4 as many hero kills)
This is true to some degree for all missions however, it’s just the most egregious for defense oriented mission. I did not lose a SINGLE unit in ANY northrend mission in the human campaign, except a few merceneries in the ships burning one. The human campaign is relatively easy other than mission 5 normally, but this was an absolute joke.
Mal’Ganis barely sends forces after you. In the last mission, you see his wave just chilling at the edge of your screen… standing still… not attacking… go play it and check it out… on the right hand corner of the screen as soon as you get control, edge of green army is just afk… from the moment I started until Arthas came back with Frostmourne, undead only attacked once, with barely any troops. They’re supposed to overwhelm you and leave your defense scattered.