I think your math is right and hope theirs is wrong. A quick look and their windsor is low too... Interesting find!
Yeah but... I still maintain my bet that a little stress by underpitching with any Belgian or weizenbier yeast helps these yeasts to express more of the lovely esters and phenols... and secondly, Windsor is a BEAST that will hit final attenuation within 36 hours if you pitch "the standard" quantity, so perhaps underpitching wouldn't be such a bad thing for that one as well, maybe then it takes 48 or even 72 hours to hit FG -- oh no!
Dave, I agree that under pitching is good for some styles. I think that I've underpitched Belle Saison twice and tasted some interesting flavors (pear, pepper, anise) in the beer. But my idea of underpitching is a rehydrated 11 gram satchet in 5.5 gallons of wort.
Here's my math (yours may vary).
If you want the beer at high kraeusen to have 200B cells/liter* in 21L, that's 4200B cells. If you want to get there in 4 doublings (should be enough lipids for 4 replications without O2), you need to start with 262.5B cells. At 55B cells/satchet** you would need to pitch 4.77 satchets.
* Don't know if that's realistic.
**Lallemand says >
5 x 10*9th cells/gram, so could be more.