I have a large garden on the side of a valley. Where I have cleared scrub & weeds off of banks, I have put a post across the bank to help prevent the surface slipping down.
The posts I have used are decent sized tree branches / trunks (e.g. 4-6" diameter) and they are held in place by stakes made from thinner branches (e.g. 1.5-2" diameter). I put the first of them in during 2017 (or maybe earlier) and only one of the stakes has failed.
All the wood came from tree cutting & was untreated. So I suspect that treated wood will last quite a few years.
Also he did not bury the shed in the ground, just a wooden support for it. I suspect that this support is sacrificial, if it rots in 10 years it can easily be replaced.
BTW, wood is timber. Timbre is the sound quality of something.