Agreed their privacy policy is not good and the owner was on record as being anti-privacy some years ago.
There was an excellent site that analyzed the privacy policies, public statements made my employees, owners etc, their history, organizations or donations that companies supported in terms of privacy and integrity. Mailbox was in bottom quartile of the list.
Sadly the site is gone but it was excellent, multiple page analysis of each provider.
Technically however it is maybe the top provider based on the excellent disroot analysis but disappointing for privacy.
I would assume batteries. They generate excess solar energy during the day and store it in the batteries for night, but that's just my thought process. No idea on what they actually do.