So is Onni jumping off that cliff in the dreamworld?
Or is Tuonela supposed to be a physical place, which he knows how to find?
I believe the cliff is in the real world, seeing Onni's clothes and weapons.
As some commented, in Finnish tales there are special places called "Lovi", usually at the bottom of lakes, where cracks between the real world and Tuonela exist, allowing those with the right knowledge (and lots of courage) to cross. If Onni physically crossed the gap to another realm or if it was just his soul (while his body is in the lake's water) is something that we hopefully will find out in the coming pages.