This is a rebranding yes.
VSTS was a bad name that confused potential customers because they thought of the IDE, and so no one really knew what the product was from the name alone.
Hah, long time TFS and VSTS user here - I recently got so annoyed with colleagues mis-using these acronymns that I sent out an email with a glossary of terms for TFS, VSTS, TFVC, VSO etc :)