No, it really isn't.
The ad aired during the debates, but ABC/Disney themselves only control some of the ads during that time period. They also allow the local affiliates to sell some advertising time during primetime. And those local affiliates are what Sinclair has been buying over the years. For example, the ABC affiliate in Washington DC is apparently owned by Sinclair.
Edit: The test as to whether it was affiliates or ABC themselves/Disney is whether everyone everywhere who watched the debates on TV saw that ad, or if it was just people in certain markets. And from what I've heard, it wasn't everyone everywhere.