Professional data recovery services with years of experience, e.g. like Ontrack, CBL Data Recovery, SalvageData, ..., are usually able to recover data even from dead drives. The really big ones have large stocks of spare parts for all kinds of old and current harddrives. If you really need the data and are willing to pay for it, those companies even disassemble the platter stacks and scrape data from your individual platters with specialized equipment. But this will cost you a few grands...
there is also the trust issue. they're going to know you have a bitcoin wallet on the hard drive so if they recover that data, they might get your private keys and cash out. and you can't prove anything. just circumstantial evidence.
I'm no expert in this field and I have also doubts that data recovery from a drive that has been exposed to nature's elements for years has any chance of success due to corrosion and moisture doing their nasty work of destruction.
with so much money and risk at stake to do a project like that, you would think they would do a test run where they bury a hard drive in some waste underground and check on it every year to see if the data is still readable. but i guess they forgot to do that and jumped to the conclusiion.