In matter of sales well yeah they are less popular than multiplayer games, but if we talk about quality, no, single player games are awesome those days.
Uncharted 4 (can't say it sells because the multiplayer mode)
Prey
Persona 5
Resident Evil VII
Zelda Breath of the Wild
Mario Odyssey
Horizon Zero Dawn
NieR Automata
Cuphead
Nioh
Wolfenstein II
Torment
Gravity Rush 2
And so many more.
The problem is that those guys are too much focus on their "field of knowledge" that they don't even watch around, and they don't see what gamers like to play. Maybe they try to convince themselves.
Well maybe he's talking about numbers, sales, and $$$, then yeah multiplayer games make more money, of course, with all those lootboxes, season pass 1/2/3, maps for 10$, etc...
But to me it's like saying that because Madonna and Beyonce sells more than great indie bands and other musicans, then those musicians don't exist anymore.