1. Only if relying upon a very peculiar understanding of "land-side" at airports can it be said that a traveler arriving into an airport after an international flight is always "land-side" whenever having to re-clear security (at a transit airport) after arrival -- regardless of the paths taken for re-clearing security at an airport -- to make an onward flight.

2. Only if relying upon a very peculiar understanding of "air-side" at airports can it be said that a traveler flying out of an airport after an international flight is always "air-side" whenever not having to re-clear security (after arrival at a transit airport) to make an onward flight.

Fortunately, neither a peculiar definition of "land-side" nor of "air-side" changes a thing for the threat-starting FlyerTalk member taking the the Delta flight out of MSP, as this journey combining Icelandair+Delta via a transit at MSP goes like this regardless of peculiar definition for "land-side" and/or "air-side":

1. clear passport control, collect any checked luggage, and clear customs at Humphrey/Terminal 2;
2. exit into the general public area land-side at Humphrey/Terminal 2;
3. go outside and head land-side over to Lindbergh/Terminal 1 (typically using the tram/iight rail type connection);
4. do the post-US-arrival passenger security screening at the the land-side general passenger screening checkpoints at Lindbergh/Terminal.

If the OP were to be coming into Lindberg/Terminal 1 from KEF -- which is where the DL flight from KEF has been coming in -- then there would be the option of a special/transit passenger security screening checkpoint not accessible to the general public. But since the OP is coming into Humprhey/Terminal 2 -- as that is what the Icelandair flight at MSP uses -- there is no special/transit passenger security screening checkpoint of relevance.