I guess that Galen is being humorous, satire or sarcastic here. That is has an awareness of what confusion his readers would have at this (not to be taken especially literally) order.
I'd say sarcastic, but I'd say the sarcasm is more in the context than in the preposition. And I don't think there's any question of reader confusion going on here.
Here's the context:
On the Natural Faculties is about the powers (faculties) and instrumentality of reproduction, growth, and nutrition in living entities (plants, animals, man). In man, the three principal powers are distributed to the body from the liver via veins (so Galen and the state of the art and terminology at that time), from the heart via the arteries, and from the brain via the nerves. And more narrowly
Nat. Fac. ends up being a lot about the body's "tubes" and the attractive powers the body is endowed with by nature.
These tubes participate in the process of alteration of the bodily substances (e.g., how liquids we drink become urine). Chapter 12: According to Galen's role model Hippocrates, substances are "artfully" designed by nature to be subject to alteration; this is opposed to the atomistic, Democritean concept of matter. (According to Galen, a good doctor must also be a philosopher!)
Chapter 13 continues the polemic, this time directed against the teachings of Asclepiades of Bithynia as representative of the atomistic view. Doctors of this sect are compelled by their doctrine to ignore what's right before their very eyes: ὅσοι γὰρ οὐδεμίαν οὐδενὶ μορίῳ νομίζουσιν ὑπάρχειν
ἑλκτικὴν τῆς οἰκείας ποιότητος δύναμιν [this is the crux of the matter], ἀναγαγκάζονται πολλάκις ἐναντία λέγειν τοῖς ἐναργῶς φαινομένοις). Case in point: The role of the kidneys in the production of, and the role of the tube called the "urethra" in the passage (think ἑλκτικὴν δύναμιν) of urine to the bladder. Not only Hippocrates and most other eminent physicians, but even butchers infer their use and function from their position in the body. In fact, that even sufferers of pains in the loins accompanied by stones in the urine infer this is shown by the fact that they call themselves "nephritics."
But Hippocrates' reasoning and theory is more magnificent than this and went beyond inferences from the externally visible, so Galen goes on to explain in great detail, mixing in anatomical observations of his own (Galen reminds us that physicians in the time of Hippocrates did not practice human dissection). Many arguments later, Galen concludes that Asclepiades is a liar!
So there's a lot of sarcasm in Galen's polemical works and in this one in particular, and the specific as well as broader context of καὶ πρό γε τῶν μαγείρων is sarcastic, but I read καὶ πρό γε τῶν μαγείρων in and of itself more as a sequential step in the argument, though indeed perhaps containing a whiff of the surrounding sarcasim. The real sarcasm is that about to be directed physician v. physician at the reasoning of the Asclepiadic sect.
So hey, ἑκήβολε, are you currently reading
Nat. Fac. in its entirety, as well as perhaps other works of Galen?
.