That pitch is going to end up costing you more money, or the player. In negotiations as a club you try to stay away from what other players may or may not be earning (no one is really supposed to know, remember?)
There are actually a lot of clues in the article that started this thread. I can guarantee you that in some point in this hypothetical pitch we are discussing, probably fairly early on, the club is going to start talking about the next few seasons, how Nick can be a part of it. They are going to talk about the players already contracted (which is where signings like Thompson come into it), why do you think CHN, DWZ, Jacko, and now Thompson are the players the club have contracted beyond 2021? What do they have in common? They are players that other players want to play next to. They are all leaders. Then they are going to say they like Nick, he fits in well, he has shown he has the ability to make it in first grade, but the cap means they have to be careful. They will then make an offer somewhat below what the agent initially asked for. Will Nick take the offer? A lot of it depends on that pitch, and how much Nick wants to stay.
TL;DR Retainment is not a yes/no proposition, it often involves a lot of negotiation which can cover plenty of ground. Having Thompson on the books gives the Dogs a point of negotiation they didn't have before.