In English or England several centuries ago if not half a millenium, 'dry rot' was a 'new' English word 'created' by carpenters then to describe external conditions on the surface of a processed wood exposed to elements like rain/snow and sunlight that has lost its structural strength.
This type of 'dry rot' wood was typically not advocated for continuing use or reuse.

Online info from tyre manufacturers and so-called tyre experts/practioners across the world always recommend replacement of a used tyre that has a so-called 'dry rot' phenomenon (as pictured above in post #1) i.e to terminate its continuing service -
implying a tyre with a so-called 'dry rot' surface or phenomenon has lost its structural integrity or structural strength.

Interpreting the so-called 'dry rot' tyre surface above as having lost for good its structural integrity/strength is imho without basis or facts, as rubber compound in NOT a load bearing (or structural) component unlike nylon/polyesters canvass or steel belts within a tyre -
It is a common recommendation or perception spread over the internet.

Hence, popular recommendation or proposal for terminating continuing service of a so-called 'dry rot' tyre pictured above is, I believe a myth and mistake - for the structural integrity or structural strength of this so-called 'dry rot' tyre is intact and is NOT lost as we are led to believe.

In a way I humbly concur with the seller and his statement, then  on tyre age.

post your own pictures pls...

compare here... not some random pics...

All my Michelin XM, Primacy and PS have suffer from this issue.