*spoilers*
Got in the tunnel! I tried calling the bird over to ride over the waterfall-pit but no luck crossing yet. I did potentially discover a bug. Upon returning to PUIJJO from the HUOM screen, the top bird was permanently gone (see imgur link). I'll see if it's reproducible.
There is a little "air shaft" in the secret tunnel that leads up....maybe I'll jump up, land on the bird, and jump again to the screen above PUIJJO. To be determined...
https://imgur.com/a/M8iEJWx
I take back my comment about the controls. After playing more and getting the hang of things, I think it's at a good level of difficulty. Since there's no enemies, there should be a challenge somewhere, and completing some of the harder jumps is definitely rewarding!