Piekoff refers to the original statement as a generalization. “A bloods are compatible.” Later they discovered an exception to that generalization.
To this day, “a bloods are compatible” is a true statement, corresponding to actual reality, but it’s a generalization – a low resolution statement. To clarify it, we would have to say “a bloods are compatible except…” or “a bloods are compatible unless…” or “a bloods are compatible on the condition that…”
The original generalization becomes a lot more bomb proof if it’s worded as “a bloods have compatibility” but in either wording it is a generalization corresponding to reality.
The child’s first discovery of a helium balloon does not invalidate his generalization that “things fall down.” Things do fall down. A bloods are compatible.
Piekoff’s point here does not amount to divorcing truth from reality. Knowledge proved incorrect was always incorrect. Knowledge proved incomplete was not necessarily wrong.