I can assure you that you are incorrect. I am the primary source and very aware of Parse's scale.

I worked on building a feature that allowed Parse to list that many databases (it had response that does not fit in the document size limit of 16mb -- requiring multiple cursors, command cursors).

I am a collaborator on the parse-server project, love the Parse team, respect what they built, and have no incentive to deceive you.

I worked at MongoDB for 4 years in various roles, consulting, engineering, etc and saw all sorts of deployments of various shapes and sizes.