Exploitation is not about whether each party is making a lot of money, it is whether one party makes a disproportionate amount of money over the value the other party produces.
I think the word "exploitation" colloquially has the connotation of exploiting the poor as they cannot afford to leave the exploiting conditions.
Exploitation is labor is never usually headlined in the context of $400k/year salary. Technically, you're right. But we can take this to the limit, instead of $400k, let's say $4M/year for a job that's worth $5M/year? Are they "exploited". Yes, but not by the common use of the term.
Perhaps the reason for GP's take was that it robs away the colloquial definition of exploiration in an entirely different context where it's the least meaningful, i.e. exploitation of Bangladeshi child laborers vs. a programmer earning $400k year salary.
No, they are talking about the Marxist theory of exploitation, where excess profits belong to the workers. It is however not a very useful model in the real world when market principles like supply and demand have much more predictive power.
