While this is technically accurate, it’s also rare. They do not oversell J as a matter of course; it happens sometimes — quite infrequently — and is usually due to a plane swap.

However, it doesn’t sound like J was oversold in this case. It sounds like Y may have been oversold, and when OP was removed from the flight, it opened a seat which was then taken by an upgrade from Y.

Knowing that OP was in F, which was not included in the original post, makes an intentional IDB even less likely than it already was. Someone at UA — either a gate agent or a computer system — marked OP as a no-show.