Hello,
First, emails are disabled by default. So, unless you activate them, customers will never receive emails from MyRewards.
Secondly, MyRewards doesn’t process any past orders when you activate it.
Processing past orders is a manual procedure that you have to perform in the plugin’s system page. Even then, you have a warning and a double confirmation before being able to run it.
Finally, your customers won’t receive emails about points. MyRewards can only send emails (after activating them) when customers have unlocked a reward, not when they earn points.
The user who wrote the review activated emails, ran the “process past orders” procedure and ignored the warning. At this point, yes, his customers unlocked many rewards and lots of emails where sent.