Am I understanding correctly that this Nobel prize is for work that was completed over 20 years ago? I'm not a biologist but it sounds like they discovered regulatory T cells together, which sounds relatively major. Is it typical for a Nobel prize to lag that kind of discovery for decades? Or is it only now that we understand how major the discovery was? Or maybe I'm just misunderstanding the discovery and the timeline.
At least in Physics, on average every year there is more than one discovery that is worth a Nobel prize. So there is an increasing backlog of people who should get a Nobel prize. You can look at the list and check that people in the 1920s got their prize about 15 years after their work [1]. But recently people have been getting it about 30-40 years after.
[1] https://en.wikipedia.org/wiki/List_of_Nobel_laureates_in_Phy...
Highs had a delay of 49 years from paper to prize, though he got the prize the year after his theory was experimentally confirmed.
Yes, many Nobel prizes are awarded for work that was completed decades ago in part to ensure that the work passes the test of time.
They got Nobel prize because they made most important discovery than all other living scientists.