The sentence is absurdly excessive BUT I don't think posters here understand why what they've done is so bad - and it is.
Archaeology is a weird paradoxical thing because inevitably you are destroying the context of whatever you dig up - you can't really do the job twice. In the past people were only really interested in treasure and shiny shit (and solid stone / sculptures/ pottery at a push) because you could see & feel it. Modern archaeology can now use carbon dating, DNA, new sorts of imaging, blah blah blah to find out much more interesting stuff than just 'nice pieces to put in museums and look at'. But lots of the most interesting evidence is organic and decays/is ruined easily- one touch of a hand, never mind a spade, and you've fucked it FOREVER. Future generations won't have the chance to test that material with amazing science because you've already contaminated or ruined it.
Even the most 100% well-funded academic or professional egghead dig is always weighing up that ethical dilemma - is it worth it or should you just not dig at all? Most of what we don't know enough about, about how people lived in the past, is about materials like food/seeds/bones/wood/straw/textiles/reeds - never mind human hair and teeth and turds - and no detectorist is going to know how to remove that stuff securely, store it properly, or do any of the follow up science. THAT is the 'lost knowledge'. The greedily-hoarded cash is kind of the least of it.
Plus that the finds are going to end up in the private collections of rich hobbyists rather than being viewed by - or even available to - the public. As supposed history fans these two should (and I would bet DID) know this, as well as knowing the pretty generous terms for turning it all in, which is another layer to their selfishness.
TL-DR: 10 yrs is a travesty but they have committed a crime against humanity - seriously.