This is exactly what I was going to say -- but it is worse! Since they (2017 song) already claim a violation, they are essentially admitting to a violation (except, on themselves!)
I would actually consider doing this if I were in the same position. You'd have to wonder what the consequences were, considering that by filing a claim against someone you are establishing that you believe there to be a violation. If the claimant then is shown to have the newer work by several years, I wonder how such a claim would be taken?
DMCA would be establishing that you believe there to be a violation, but ContentID is just automated matching YouTube does on the behalf of artists - so no such claim exists.
surely, by enrolling to have ContentID match a work which you claim is yours against others, you are initiating the chain of events which leads to the automated claim of infringement being made? This implies (to me) that you are the one making the claim?
That's exactly what I was thinking: by making the claim they've admitted that both the number and your video are using the same music. Only you have proof that your video existed first, ergo they stole your work :)
