Really, it isn’t a coherent category, of course, but if you look at the authors he lists there, you can absolutely find the elements of ideology, misconstruction of class antagonisms, and their role in the perpetuation of exploitation to unify them.

To add: a large part of what he passes for "advice" (ideology would be far too generous in my opinion) is based on two principles, which are actually one derived from the other:

• individual responsibility and exerting power over one's own 'sphere of influence' are the only explanations for any and all human interactions and activity. The individual stands at the center of a chaotic universe, and is the only way through which said universe is able to bring order into it.
• systemic oppression and the individuals/structures that uphold it aren't real, since only individuals and their choices are what can materially affect their own lives.

This means that any person who has identified or analyzed the role of systems and structures over human agency and society is a Marxist, because they deny the might of individual action, and -according to Peterson- offload all responsibility onto the "better" individuals. When systems and contexts are incapable of restricting human agency, then any conclusion to the contrary must be made out of envy.

By that logic, even philosophers predating Marx are Marxists.