It is a fair point, but I suspect the methane from shale is way way older than the permafrost methane. My understanding is that the carbon isotope half life is ~5000 years, so after 50,000 years it is 0.1% of its starting level and becomes useless for determining age.
the first 460m of permafrost depth have formed over the past 35,000 years. The permafrost that is thawing now and releasing CO2 has been formed during that period. There is deeper permafrost which may be much older, but that isn't thawing now and releasing CO2.
According to this table: https://en.wikipedia.org/wiki/Permafrost#Base_depth
the first 460m of permafrost depth have formed over the past 35,000 years. The permafrost that is thawing now and releasing CO2 has been formed during that period. There is deeper permafrost which may be much older, but that isn't thawing now and releasing CO2.