In many airports, there is a buffer zone after you clear immigration, pick up your checked bag and clear customs. Technically you are in the country but it is not open to the general public - a good example is the bag recheck area after customs in the US - people meeting someone arriving on an international flight are not allowed in this area. I have come across similar zones in other countries - once you exit through the double doors, you cannot re enter that specific area.
So, my concern was, exiting the customs area and finding myself in a similar zone where the Polaris Arrivals lounge is. By leaving it to go and check my bag at the Icelandair counter, I could find myself unable to return to that particular section of the airport.
I am aware that United doesn't care about my flight with Icelandair & will recognize me as an arriving Polaris passenger, I just want to make sure I can enter that area again after I've checked my bag.