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.

I cannot agree with you here.
First, the hdd is not operational so the life is much longer than only 9-20 years.
Second, the important thing here is the data stored on the platters, the condition of the platters is crucial not the drive itself.
Third, the HDD cast body is from Aluminum Alloy A380, used in the aviation, hard and corrosive resistant, the same valid for the top cover, rust free and reinforced plate and the rubber gasket on the plate has more than 10 years of life.
There is a small ventilation hole on the top cover with filters, so the HDD can "breath" when the air pressure changes like on higher altitudes but even if there's corrosive fluid leaking into the drive which is highly unlikely, the platters are made of aluminum or ceramics covered with magnetic "dust" which is coated with extra layer of carbon.
There are probably mechanical damages from the transportation to the final point, but since the drive was not in operation the heads probably hit only at one specific spot on the platters, creating physical damage.
I assume the guy threw his whole computer with the mounted HDD inside, if that is the case then the chances to recover the hdd are much higher.
If I was him I would Dig!