Yes, it was overbooked, and there have been mulitple cruises overbooked this year. Actually the system has worked well and Regent offers a combination of benefits to the guest to voluntarily bump. These may be a full refund and cash or a full refund and a shipboard credit, a substitute cruise or cruises that are usually much longer, often in a large suite.
How they select those people to offer the voluntary bump to I do not know and this could vary from cruise to cruise depending on how much additional space they require.
This particular sailing had some wonderful offers. If a person is flexible with their vacation plans a lot of guests are extremely happy with the situation.