Not sure my word is going to mean much, but yes, we do indeed plant these trees :) We also publish regular tree updates to make the impact more visible (latest one is here https://www.youtube.com/watch?v=FeBhY5m9npE&) and just now some of my colleagues are in Indonesia: https://www.facebook.com/ecosia/videos/1056643057857650/

We also use a ODK-backed system for monitoring, where our partners on the ground fill in surveys about the trees they planted.

I've seen Pornhub doing similar initiatives. Haven't read any updates whether they did plant all trees that they promised to. No one keeping tab.

Seems they are a certified B-corp (just looked it up, seems legit).

Here is another reference: https://info.ecosia.org/about they have a pretty sizable team. Pretty cool!