I think a better metric is how fast I'm switching between things. Logs, Slack, editor, something else: if the monitor's flashing like crazy as I go back and forth trying to tie things together...
It's certainly not something we all can agree is wrong. For me, the author's metric is reasonably accurate. If I'm typing, I am almost certainly in the zone and I will lose it if someone disturbs me.
It's definitely rough regardless (I spend non-'flow' time in the editor) but people are largely objecting to the perceived idea that it's necessary to be in the editor.
I don't know if this is a good measure or not, but it's fun to build and use.
I'll try it for a few months and maybe post my thoughts.