Yeah, the manager fucked up by providing too much info and wording it poorly. If it’s “you two have different skills and contribute to the team in different ways” that’s another story. If Marcus is also making sure the rest of the team feels supported and keeps morale high then that is work too! (As someone who often fills the emotional support human role, on top of regular duties, I’m glad that this could be recognized). Productivity is more than work output, and comparison is the thief of joy.
If the person is upset because they feel their workload is too high, or that they aren’t being compensated for their own work, then that’s a different convo they should be having.