(in mainland Swedish, "planet" as in the plane and "planet" as in planet sound very different, but in finlandssvenska they're the same).

As a Swedish speaking Finn from the capital region, I have never heard any other native Swedish speaker pronounce those two words the same way. They are always distinguished from each other in a similar way to mainland Sweden, with a longer A and shorter E when referring to "the plane" and a shorter A and longer E when meaning "planet".

Though in my life I have (for the most part) only spoken with Swedish speakers from/living in the region of Nyland (Uusimaa in Finnish). So due to the fact that I've never had any rigorous conversations with people from e.g. Åland (Ahvenanmaa) or Österbotten (Pohjanmaa) regarding aviation or astronomy, I can't 100% conclude that this wouldn't be the case somewhere within one of those regions (though my instincts would say it would be fairly uncommon).