This is less about playing mindtricks and bluffing, but more 'mundane'.
I.e. a decent player will know all the good mainlines of popular openings, and end up in 'comfortable' positions (among other things due to computer analysis).
The metagame is to prepare a non-garbage sideline, that your opponent is not so familiar with. Nobody at a high rating plays 'questionable' variations on purpose, in order to bluff. The resulting positions would be much too punishing.
Post-game thoughts from the players:
Inteviewer: "Let me pull you back; so you didn't understand the position, and so you still felt like you were scared to go into a piece-up situation?"
Alireza: "Yeah so, I just trusted him. (he shrugs) I just wanted to make a move.. and play a bit more you know (laughing)"
https://www.youtube.com/watch?v=xT9orNSgk5w&t=55s
Niemann: "You have to understand, when I play the move Qg3, this is a purely pyschological move."
https://www.youtube.com/watch?v=CJZuT-_kij0&t=780s
Whether or not Niemann did cheat over the board, this shows that even top players can be susceptible to this kind of bluff.
This is why playing against a maniac (seemingly random and not respecting bluffs or equity) is very difficult because if they get lucky enough times, they are able to "break the game" by getting the opponent to be extremely risk averse OR take on more risks.
I believe this is what Magnus is referring to, its that making yourself unpredictable by questionable moves and no longer playing in a way that has been taught.
For example the common strategy is to go all in with strong pairs like AA, KK but someone beats it with a totally random garbage hand (52o, 37o) and does so repeatedly, no theory can help you win against somebody who is just repeatedly lucky and brash.