Kay Sievers' Wikipedia page cites a blog post by Lennart Poettering [1] which says that systemd was designed in "close cooperation" with Kay Sievers and that Harald Hoyer was also involved, so it seems pretty clear that he's on the team that develops systemd, the team that Harald Hoyer referred to as "our team". All three of them gave a talk [2] together in 2013 about what they were developing.
If Lennart Poettering "adopted" varlink, he seems to have done so from members of his own team ("our team") who created varlink and who are also fellow co-creators of systemd.