He cited Justice Story to support the conjecture.
I can make similar statements regarding UFO's, talking about a growing body of scientific commentary, and early hisorical writings, and even a quote from Art Bell. Does that mean I believe in UFO's?
That said, Justice Thomas may indeed consider the RKBA the key liberty of a republic -- it's just that he didn't say it, that's all.
If someone said, "Marshaling an impressive array of evidence, a growing body of scholarly commentary indicates that UFO's are, as the evidence suggests, real", then I'd bet a hefty amount he believed in their existence.