That is true. I have studied that thread since my Cannondale is stuck at 126mm. While there is some benefit with the spacers, the real width savings is in the locknuts. I have collected a few very thin locknuts for this purpose.
I had planned on using a FH-7700 and reduce the OLD close to 126mm. From comparing FH measurements, I believe the 7700 matched the measurements of the DA FH-7403; but it has been a number of years since I looked into this. Also the locknuts on the 7700 are very thick.
In the end, I swapped out the 9 speed 7700 titanium freehub body for one off an XT 732 and I’m running a 3x7 which is serving me well.
John