Technically, his behaviour is against Medium's rules:

https://policy.medium.com/medium-rules-30e5502c4eb4

"Performing a disproportionately large number of interactions, particularly by automated means. This includes bulk or indiscriminate interactions, such as following of other accounts (follow spam) clapping, highlighting, or leaving notes"

By highlighting everything, he's getting his name seen all the time and getting us to write about him. And it increases the number of notifications we get. If everyone highlighted like he does, Medium would be unusable.