Denny, where could I read Mr. Caldwell's statement about this, because I feel like I'm missing something? While I don't doubt that the sensor glass effects the light path, one would expect that Blackmagic already compensated for it's effect, and I can't see how it would act differently for a lens vs a focal reducer. If we look at Metabones BMPCC speedbooster compatibility chart at the bottom of their press release pdf they show all of their micro four thirds speedboosters work with the BMPCC, but just offer different multipliers.
https://www.eoshd.com/uploads/metabones-sb-blackmagic-pr.pdf Also notice the BMPCC speedbooster says it won't work with other micro four thirds cameras, but the reason is because the rear flange of the .58x speed booster is so long it risks hitting mechanical bits like the shutter, or producing vignetting if the sensor doesn't crop enough. That said users have successfully used the BMPCC speedbooster on a GH4 in 4k by locking the shutter open, but they had to be very, very careful in how they set up the camera. Both these factors suggest the only focal difference would be the multiplier & glass of the reducer.
With the above said Metabones does have an adjustable rear element so users can compensate for an older lens whose elements may be out of alignment. I do believe this used to be a unique feature for the speedbooster, but recently other focal reducers like the Viltrox NF-M43x have added a similar adjustment feature.