I seriously doubt that the data will still be there to be retrieved, because under
ideal conditions, hard drives are predicted to be able to retain their data for 9 to 20 years. So I should say... a landfill are less than ideal conditions and his hard drive has been there for almost 8 years now.
We have not even consider that corrosive fluids will hasten the deterioration of the physical disc and also rust and the pressure that was placed on this hard drive with material that was placed on top of it.

Actually, the possibility of successful data recovery depends entirely on the amount of damage to certain files.
Even if you accidentally delete any data, it is possible to recover the data through forensics unless the data in that specific block where it was, has been overwritten again and again.
No problem if your HDD is old, if not overwritten then there will be a copy of your file from year to year. But once corrupted by overwritten, retrieving old data from that block is almost impossible.