The question is, who are you being authentic to? The person you're becoming or the person you currently are?
Both are versions of authenticity but only one is useful.
Most likely, the "not useful" one feels more natural at first, but that's just because it's the one you've practiced the most.
The "natural man" or your mind on default is not your highest self. And only by practicing integrity and authenticity to the highest version of you, will you become it!
There's no reason to call others inauthentic for trying to be someone they aren't YET. Let them try, let them learn. Let yourself do the same.