No, it wasn't. It may have gone from being influenced by SGML to being defined as an application of SGML by the time of the first spec, but TimBL didn't actually originally use SGML for it due to implementation complexity of SGML.
TBL might not have used SGML tools, but surely wanted to make HTML extend to SGML proper and appeals to SGML concepts at several places in the linked document:
> Currently HTML documents are transmitted without the normal SGML framing tags, but if these are included parsers will ignore them.
> In SGML terms, paragraph elements are transmitted in minimised form
> These tags are kept as defined in the CERN SGML guide. Their definition is completely historical, deriving from the AAP tag set.
No, it wasn't. It may have gone from being influenced by SGML to being defined as an application of SGML by the time of the first spec, but TimBL didn't actually originally use SGML for it due to implementation complexity of SGML.