The test reports do not account for diffraction. They only look at the wavefront of the telescope. They typically measure hundreds of distinct points on the lens (or mirror) and derive a Sthrel figure from the deviation of perfection.
Now let's look at the wisdom of that. If you really want to know what the quality of manufacture is, the size of the aperture and the presence of a secondary obstruction don't matter. You are only concerned about how the wavefront converges on the focal plane. It either propagates perfectly or it does not.
As I mentioned earlier, the polychromatic Strehl is not really influenced so much by the manufactured quality. In a refractive optic, it is set by the types of glass used, curves in the glass chosen by the designer, and the spacing of those curves. This is what sets the polychromatic Strehl. No amount of precision in manufacter can raise this number. It is not usually included in bench tests though because again, the interferometer test is really just looking at how well the curves were generated. It does not really know where the different focal points of the light are. Now if you tested in several colors you could derive the polychromatic Sthrehl, but why would you do that if you were really just looking to see how well the scope was made?
To the OP though, your question was excellent.
And here is the hypocrisy of the refractor forum. I have seen this many times over the last 20 years.. Someone in the refractor forum will complain that Strehl testing of reflectors does not really tell the full story, because well, you know, if you included the obstruction, the Stehl of a C8 would never be better than .8 so they say that all C8s have a Strehl of .8 at best. (And of course this is malarky because the Strehl value is as I have repeated several times, a mesurement of optical quality and optical quality alone).
These same people though simply choose to ignore that when CA and Spherochromatism, they have the exact same result as a secondary obstruction, which is that they lower contrast. As the tables I referenced above show, a 100mm f/10 achromat has about the same contrast loss as would be experiences with a 100mm reflector with a 30% obstruction.
People can demonize the contrast loss of a secondary obstruction but give a total head in the sand pass on contrast loss caused by CA and spherochromatism, which can both be quite bad in large, fast achomats. By comparison, the contrast loss of a secondary in a marvelous scope like the Intes Micro MN66 is almost nothing, being less than some of the fast ED doublets out there.
Anyone that is a refractor lover and that thinks that a secondary obstruction is the end of the world should beg for a peek though an MN76 if they ever get to look though one at a star party. Spectacular telescopes. Most people on CN will never own a refractor that will beat an MN76 on contrast. Heck most will never own a refractor than can beat an MN 66 on contrast. Buy comparisn to CA in many scopes, a 17% obstruction is nothing.