Hope 1 said...
I have always understood there is no such thing as zero PSA and any reading under .1
At a high-level, this is also pretty much what Prostate Doc is telling you. That's usually enough information for the general population. If you want to know more, then read on...but if that's enough information for you, then stop here.
There are now two types of PSA tests: the "standard" PSA test, and the newer "ultrasensitive" PSA test.
The "standard" test has a lower detection limit of 0.1 ng/mL, so if (as you said) a reading is less than that threshold, it is reported as "undetectable." There really is no such thing, also as you noted, as zero PSA in a living breathing man even after RP, but the amount of PSA may be so low that it is undetectable in the "standard" test.
The "ultrasensitive" test typically goes down to a lower threshold of 0.01ng/mL (or a little higher, or lower; there are several types of ultrasensitive tests, but this is the most common one). After RP, there may be trace amounts of benign tissue left behind from the prostate that still gives off trace amounts of PSA which MAY be measurable with the ultrasensitive test. This is certainly a detectable amount using the ultrasensitive test, but would have probably been undetectable using the standard test.
Hope this further explains...if you wanted that additional info.