This is interesting, and I personally feel he is fighting it only because it buys him more time. In a different article (linked in this one), where they announce Alabama’s plan to use nitrogen it says:
Smith, in seeking to block the state’s second attempt to execute him by lethal injection, had argued that nitrogen should be available.
So he literally asked to use nitrogen, they said “ok” and now’s he’s saying “how dare you try to use me as a guinea pig”