Shamrock likely
isn't Supreme Commander of God's Knights based on his title. Compare his title (this chapter) with Garling's introductory title below (Chapter 1086)
The term "
団長 "in "神の騎士団
団長" (the rest just means kami no kishi-dan aka god's knight) can mean "leader", but leader as in
commander of an individual unit when used in a military context
meanwhile the term "
最高司令官 "in " 神の騎士団 の
最高司令官" explicitly means supreme commander, one of the terms used in japanese for the person at the top of a military, with authority over all units (used for garling in his pre-gorosei appearance in chapter 1086).
(both images taken from japanese-language wikipedia, and while it is wikipedia, these are definitions for highly publicized terms with examples that are pretty easy to verify independently with other japanese-language sources, and these definitions definitely seem to hold up with the other japanese to english dictionaries and resources ive seen online)
At the bare minimum, the title Oda uses for Shamrock as a leader is completely different from the one he uses for Garling, and so that is noteworthy by itself.
Also not hating on the idea of him being the primary leader/supreme commander either, if that's the case it would be cool too, but I did wanna share this information because I think it's important to know. I'm excited to see more of this guy going forward regardless, and hopefully TCB and Viz will make this detail clear too