I think what we're seeing there is that the guide rails prevent the flanges on the slightly narrower Swedish wheelsets from moving sideways far enough for one of the wheels to drop off the wider Finnish rails. There's not a huge difference in the gauges, and by forcing the narrower wheels to run in the centre, they can run on the wider rails, on the outer edges of the narrower gauge wheels.
I think it means you can run Swedish wheels on Finnish track, but not Finnish wheels on Swedish track.
It reminds me of a real 'sit-com moment' in France in 1990. We were starting a tour of the chateau at Amboise and the guide was the most multilingual person I've ever seen. She was dealing with a barrage of questions in many languages, from all directions and handing out leaflets for the most common ones, English French, Spanish, German, Italian, etc. A chap standing next to me said something totally incomprehensible, on the off-chance that she could speak his language - it was clearly one that she didn't, so he reverted to a heavily accented English question?
"Do you have those leaflets in Finnish?"
Straining to listen, but not understanding most of it, she realised that the question was in English and guessed what he was asking, based on the word "Finnish" - unfortunately, she actually heard him say "finish" in her head and said - "About forty five minutes."
This mystified him, which then mystified her and I started laughing.
I nearly had to draw diagrams to explain to them what had actually happened...