Because to reach the IOM you need to start in the UK so should people want to get there they would still need to fly to UK.
This edition of High Life also had articles recommending Puerto Rico, Anguilla and Jodpur despite BA not flying direct to them.
Looking at the Emirates Open Skies magazine now too they also have ads for plenty of destinations they don't serve - Antalya, Sulawesi. And United's has features advertising destinations in Sweden and Santorini despite no UA flights.
Qantas has features on Oslo and Austin and adverts for cruises from Greece. Essentially its pretty common to have features or ads on destinations you perhaps don't serve direct. and a great way to raise awareness of a place beyond the usual suspects which we seemed to conclude the IOM needs to do.
And as I said, BA at the moment are not offering flights to Jersey from the Summer 22 timetable yet still have adverts for it.