Urgh, atmospheric ignition was considered but it was studied 6 months before the first nuclear test was conducted: [1] https://www.realclearscience.com/blog/2019/09/12/the_fear_th...

The whole atmospheric ignition thing depends on physics which just doesn't work: specifically, that you can reach a temperature without gravitational pressure that triggers nuclear fusion at ambient atmospheric pressures.

It is in the same category as the "concerns" the LHC would create black holes which swallowed the Earth.