Note: Reachable.
Most nodes are likely private and on the tor network.
As an example, look at lightning:
Discussion
I thought reachable meant that theyre actually communicating with other nodes, not that they're clearnet. nostr:npub17u5dneh8qjp43ecfxr6u5e9sjamsmxyuekrg2nlxrrk6nj9rsyrqywt4tp can you clarify?
Bitnodes.io lines up with OP. The majority of nodes (~10k) are on tor, but the total is ~16k 
That's fair.
In that case, I too wonder what the meaning of reachable is here.
Nodes that only accept connectivity via inbound requests, nodes that also initiate and establish TCP handshakes themselves or nodes that consistently remain online over a period of time.