I possess a non-trivial amount of bitcoins, I sold some. It's really impossible for me to figure out what I got those specific bitcoins for, especially since this was around when bitcoin was at around $1k a pop so selling 7 or so was absolutely nothing for me.
According to Finnish law money laundering is activity where someone receives, spends, transforms or transfers illicitly obtained (this is a bit difficult to translate but the law specifically says "acquired by crime") property to cover up or disappear the origins of the property.
Logically you shouldn't be able to be convicted of this without proof that the crime actually happened, but apparently not here. Lighter sentences seem to make it easier to wrongly convict people, especially when it's extraordinarily rare for them to actually end up in prison.