They produced killer-mentats. They'd produced a killer medic, overcoming the suk inhibitions against taking human life to do it.
a) Little bit of writing that Frank might have regretted, or wanted to explain later?
b) Suk was actually meant to be a Tleilaxu creation?
c) The medic in question is not Yueh?
d) They actually meant that Piter, a twisted mentat of the Tleilaxu school, created Yueh with his twisted ways and managed to break his conditioning, hence making post-conditioning Yueh a Tleilaxu creation, but only by proxy?
e) some other explanation?
I think d most likely, or possibly a.
If it's c, who else could it be? Yueh was the first with the conditioning to ever break it, and quite likely, the last: the suk school obviously persisted (the Rabbi and the sisterhood) but the conditioning or boasts thereof probably disappeared as soon as Paul came to power and set the records straight.
B? I guess the Harkonnens could have commissioned the tleilaxu to create Yueh for the purposes of sending him to suk school, breaking him later and then getting him sent to the Atreides somehow. That seems less parsimonious than just breaking a regular suk though.