>Well, that is just plain two round system, it is pretty common and definitely better than FPTP.

I would differentiate it from a two-round system in that voting takes place on a single ballot paper.

In two-round elections there is the ability to use the information about who is in the top 1 to cast the second vote.

In Supplemental Voting an elector has no information as to who's going to be in the top 2 when they cast the second preference.