Is it free?
Yes
Can I use it commercially?
Yes, you can use it for commercial and non-commercial games completely free!
Is it really a game engine?
Short Answer: For common purposes, sure, you can think of Monogatari as a visual novel engine
Technical Answer: Monogatari is a web development framework created to build modern websites that just happen to be games, it lacks quite a lot of elements to be considered a real game engine as it is only using an already available platform: your browser.
Who develops Monogatari?
A lot of awesome people help developing Monogatari, you can view the whole list here: https://monogatari.io/contributors