Try changing the commas to semicolons. Those quotation marks also change their format on the Inderes website, so you should try retyping them manually. It works for me with this formula:
=ARVO(VAIHDA(INDEKSI(split(importhtml("https://www.pyn.fi/pyn-elite/arvonkehitys-tuotto/";"list";1);"*";TOSI;TOSI);1;2);".";","))