IMO:
- DL doesn't need any more A350s or A339s beyond what they have on order. Both are great planes, but I think DL has satisfied its need for these.
- There is very little doubt in my mind that the Boeing 797 is coming. I'll go out on a limb and predict that it will be officially announced next month in Paris. DL really wants this plane, they have pushed hard on Boeing to make it. It really seems pretty straightforward to me that this will be DL's next order and the 797 will have an important in DL's fleet.
- I can easily see DL adding a long term order for 10 to 20 777X to get a better price on the whole deal.

Look, I have no dog in this fight, no affiliation with either manufacturer (other than I grew up right next to the Bombardier plant where the A220 is currently being manufactured). I think Airbus was the better deal for what DL needed last time. And it's fairly clear to me that once the 797 comes out, Boeing will be the better deal for what DL needs next time.

I see no drama here, just a well-run airline making good business decisions.