The quote is "Of all the souls I've encountered in my travels
, his was the most human"
In that statement there is clearly an indication of speaking as an explorer, a traveler to alien worlds. Spock is a being alien to him, & of all the beings alien to him that he's encountered in his travels, Spock's was the most human. In that context, it is not even arguable
I choose to look on his statement as waxing poetic about how Spock was a singularity in his life, an alien being who had become closer to him & more human to him than any in an entire galaxy of beings he'd encountered
It is precisely because he isn't Human that it is worth noting how human he actually was. That's the whole point. At least that's how I interpreted it, anyway