If you don't know someones gender they/them is not misgendering. (Should be the default for anybody)
If you know the person's gender and it's not they/them then it's misgendering unless it's ok with that person.
Using they/them on someone who looks trans and nobody else is just as bad as misgendering someone.
Its simple. If someone has not told you there pronouns, use they/them. If you know there pronouns then use what they want. (Yes it's really that simple)