since methane hydrates can not be formed at atmospheric?

The frozen surface of the tundra acts as a cap over the methane produced below the surface from large amounts of organic material that has been decaying in some cases for thousands of years. The thawing allows this methane to be released and accelerates the decay of these large deposits of organic material. This source of methane was never a hydrate but is simply a gas.

Methane is still produced at atmospheric pressures. It can be emissions from deep within the earth or formed from the anaerobic (little oxygen) decomposition of organic matter by bacteria. Methane hydrates are formed at the bottom of the ocean and are only stable there. They have to have a steady quantity of methane to form the very thick deposits that are known to exist.

Methane is already held in great volumes in the frozen arctic. It will be released as the permafrost thaws, which it is doing.