A direct dispute can be dismissed without need for any investigation as "frivolous or irrelevant" if the dispute is substantially the same as a prior dispute filed with a CRA. One issue, one need to investigate a dispute.
See the implementing rules for the direct dispute process, as set forth in 16 CFR 660.4(f), for dismissal of a direct dispute based on a prior dispute filed with a CRA.
They can dismiss, not because they no longer own the debt, which is irrrelevant as long as their reporting remains, but rather because the dispute is substantially the same as a prior dispute.
File the direct dispute and get their response. If they dismiss based only on having sold the debt, that is a violation of the FCRA 623(a)(8), for which you can then file a formal complaint with the CFPB for violation of the FCRA.
File a complain with the BBB and request to have this CO removed. I had it done with an unpaid charge off with them. it worked and they took it off all 3 reports.