Literally no experts have said they are confident the method is painless, though.
Just wikipedia PhDs talking about industrial accidents in vastly different circumstances, where victims were caught entirely unaware.
You're basically asking me to prove the negative - so let's put the shoe on the other foot. Find me medical experts testifying that this execution method would definitely be painless, because it's really easy to find medical organizations and medical experts saying their concerned the process could be torturous.
Dr. Philip Nitschke, a much-interviewed expert on euthanasia that shows up in a lot of these articles and who specifically studied use of N2 asphyxiation, expressed great concerns about how this was being administered. The only way this can even conceivably be administered painlessly is if you either catch someone by surprise or have their full cooperation. If they're an unwilling participant you are torturing them to death using a technique that has an indefinite amount of time to work properly. The setup of how Alabama was going to be doing this would be slow and ineffective, and all of that delay is going to be torturous.
And if you can't find that testimony that this process is definitely humane maybe you should stop assuming it is.
It's true, it's pretty inconceivable that any execution method isn't torturing someone to death. Hard to come up with any theoretical framework for that other than taking a gun to the head in your sleep. But these pseudo-scientific techniques that simply refuse to think through the practicality of administration are entirely designed to make the execution more pleasant to watch for the onlookers and that is particularly heinous.
Stop thinking about the methos you would want used on you if you had to be executed. That line of thinking isn't analogous or reasonable. Think about the technique you use on someone who doesn't want to be executed. Because that's an entirely different thing and the practicality of administering is part of how you make it more humane if that's really your goal - and to be clear I'm sure that that's not anyone's goal here.
The goal of the using this technique is being fulfilled perfectly by you, since it was to fool people into believing there is such a possibility as a more humane execution ( that was still reasonable for onlookers).