Their national identity is one of mixed Bulgar-Slavic, they usually take Christian names, Slavic names or Bulgar names, but their language and culture is indeed South Slavic. I say yes.
Indeed genetically there are clusters, but this can't be criteria to decide who is Slavic and who not. I know those stats, but then would you suggest that only East and West Slavs, Slovenes and some Croats are only Slavic, and that rest of Croats, Serbs, Montenegrins, Bulgarians, Bosnians and Macedonians are not? Remember, many Russians cluster with Finns more than other Slavs (N3).