Just a quick update: called DL to add my number to the booking. A bit odd, this was before the first leg of the flight, which was actually leaving from Europe to the US.

Checked in on the mobile app afterwards, all is well, the boarding pass that went to iPhone wallet has the "TSAPre" logo on it.

Just reporting this as a previous poster apparently only got a jpg rather than the normal boarding pass formats (pdf, mobile).

PS: first time I called DL-for-AF in the US. Pleasantly surprised that my FB number was requested, which may have moved me forward in the waiting line.