That's honestly the less important part. Even if the atmosphere were pure hydrogen (whether protium or deuterium or tritium) you would not build a hydrogen bom

That's honestly the less important part. Even if the atmosphere were pure hydrogen (whether protium or deuterium or tritium) you would not build a hydrogen bomb without its own encapsulated hydrogen fuel source. The black magic of a hydrogen bomb is that it focuses the explosive energy of a fission bomb to ignite a fusion explosion. Doing so requires a very specific configuration; any hydrogen on the outside of the bomb, regardless of its concentration, would not reach the appropriate temperature/pressure to fuse.