Almost none of that matters. You can certainly argue that Manu was underpaid in his career. It's a lot harder to argue that he took pay-cuts. He signed a six-year deal after his rookie contract was up. He followed that up with a max extension, then took a market-level deal, then he took a big discount, then he took a huge overpay.
It's not too dissimilar from Steph Curry's contracts so far. He is MASSIVELY underpaid right now, but he didn't take less. He signed a deal that made sense at the time and just massively out-performed it. Just like with Manu, Curry isn't going to give the Warriors a discount this time around.
Manu's earnings have more to do with timing than anything else.