No, he is not the sole user. There are a few of use SPITBOL users scattered around. There is even a Yahoo group: firstname.lastname@example.org (perhaps a sign of our longevity).
Looks like he’s still at it, but doesn’t currently have an ARM port: http://daveshields.me/
Also seems like he gave up on NASM and switched to gas, basically in order to get better debugger support.
Huh. I wish the guy luck!
I read this a few weeks ago and it prompted me to read around on the guy’s site and writings, and I found it all quite moving. Time marches on, colleagues, collaborators pass away, eventually nothing beside remains … Just saw it was picked by, uh, Motherboard: http://motherboard.vice.com/read/this-70-year-old-programmer-is-preserving-an-ancient-coding-language-on-github