It has you going into Costa Rica to get from David to Puerto.
Do not do that.
1. Drive to Paso Canoas (It's on the Costa Rica border)
2. Turn left (south) at border
3. Arrive in Puerto Armuelles (The road ends in Puerto & at Pacific Ocean)
I wrote a page with detailed instructions on how to get to Puerto Armuelles.
It includes what to expect at the police check points along the way.
There is a bit on buses and taxis as well.
Please comment below.