Just an educated guess but I am assuming the age of the wood is a good enough proxy to construction. Making the assumption that wood out in nature will decompose in short order (when thinking of the stated age). Being off a few thousand years is probably ok.

But somehow the wood is still intact 500k years later?

Kind of as other said, it was buried. From what I have read archaeological finds are about piecing together good guesses. Sure some wood is rot resistant but I suspect you would be hard pressed to find wood sitting outside for 1000+ years on the ground that early civilization would find appeasing to build with. Anything is possible but I am guessing it either fell natural or was harvested by those people and they decided to build with it somewhere plus or minus a thousand years.

It was probably buried under anoxic conditions, which would weaken it and make it less suitable for new construction.

"Intact enough to be recognized" is a lower standard than "intact enough to be useful as a building material".

From the Nature article posted by unwind, it sounds like they dated the sand surrounding the wood, not the wood itself.