If you define "authority" as "someone having the" right to rule".

Then all the doctors and IT specialist don't have authority.

Having knowledge in a certain field that people choose to use is totally different from having the right to rule. Giving the same name (authority) for two completely different things is only there to confuse people.

Putting the word "legitimate" for the word "authority" does not help much to bring clarity in the (Orwellian) situation. And makes it even more (Orwellian) unclear

A doctor or IT specialist has not, and can not have the right to rule you.
They are an "authority" in a certain field of knowledge, they are not "authority" over other individuals/self rulers and you can ignore everything a doctor or IT specialist has to say you must do. They can not punish or harm you if you don't follow their words. (advise or dictates)

Authority (the right to rule other people and punish them if they don't) does not exist.

I'd also like to put a distinction between what people call leaders and rulers.

Leaders ( and their advice) are followed voluntarily or not, rulers force you to do what they say.

For me ( a voluntaryist) to call leaders; "voluntary rulers" would bring the same confusion to the situation as putting the word "legitimate" for the word "authority" and call people who only can serve you (doctors/IT specialist) that can not rule you; "rulers"

a little joke
Imagine a slave who discusses with you the difference between 'iIlegitimate slavery' and 'legitimate slavery'. And to make it even more fun. He say that one is legal and the other is illegal. lol. Oh wait.