Source for that? As an experienced frequent flyer with status on multiple airlines, as well as several friends and relatives who are airline employees, I don't think that's correct in the USA. They can't ask for disability details, but they can ask if you need assistance, a wheelchair, etc... If you simply ask to board early and state that you need to store an instrument in the overhead or closet, they very well may grant your wish, but they can certainly ask why.
As an example, American now kicks people out of line if the ticket is scanned and your group has not yet been called.
You simply go to the gate agent, state that you need extra time to board, and they will re-print you a ticket with XT on it. They may ask, but you don’t need to disclose to them.
This applies only to Southwest, I can’t speak for other airlines.
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