Hmm, Delta support indicated it was a "special negotiated rate" which is why they couldn't touch it to modify it either (just kept getting errors for both cancelling and modifying). Doesn't really make sense since it was the same fare as booking directly but it didn't feel worth it to fight it. I have had a lot of difficulty changing and cancelling flights booked directly with Delta in the past year though (always have to cancel and rebook), so it may just be a YMMV situation.

Edit: Trying to use the C1 airline travel credit is annoying, I am only being shown basic economy tickets despite my original cancelled flight being main cabin. Called C1 support and he is claiming the "economy cabin" is the same as main cabin (which can't be, since C1 shows "main cabin" just fine when booking a regular flight). It shows me that seat selection is not an option which confirms to me that economy cabin most definitely is basic economy. I'm not ready to rebook today but he assured me if I book over the phone they will book my seat and ensure the fare class is correct. Lol.

Edit 2: Went back to Delta and apparently C1 did not fully cancel the booking correctly. The Delta agent fully cancelled it and now the amount shows as an ecredit through Delta. Apparently there will be a $50 charge if I use the credit directly but if I work through C1 they can waive that fee. At this point I'm thrilled to take the $50 loss if it means I can book through Delta with the ecredit. With the 10k anniversary points from C1, I still come out ahead with a $250 credit instead of $300. So at this point I'm not sure if I'm a cautionary tale, or there was just a glitch with my cancellation. I don't see a $50 fee when attempting to book a flight with the credit thru Delta (didn't go through with it though) and I still see the $300 credit in my C1 portal. So not sure exactly what will happen when I do book with the credit...could be interesting