I got one of those letters from Chase as well.
DW and I had 3 accounts between us (hers, mine and ours) with Chase. Got 3 GW deletes outta them a few months back, and they even put it in writing they were deleting.
Then I got the weird "We cannot verify as this account number is no longer in our system." I rechecked DW's and my TU reports. [TU shows the full account number]. It was never ours to begin with. I just shrugged and said, oh well.
Since you have a current GW letter in the pipeline, and I would let it lie for now. Chase will usually respond within 2-3 weeks. Give 'em 4-6 weeks to GW delete or respond and say no.
If they delete, no worries. If they say no or don't respond, ask again.
2 of our 3 GW deletes were outta banks Chase acquired. I suspect their migration of data ain't exactly a completely streamlined process, and working in IT I know it never is. They might well be losing old accounts.