The GA was trying to get you to change for free. It's not a policy what they said, but if they can get you agree to a reroute for free, it may work on some people.

You get comp if you take a VDB. Doesn't matter when you get in. You could have volunteered again on the plane or insisted on comp at the time, and they probably would have caved.

Like everything with CO/UA these days, you have to assert yourself to get what you deserve.