This is a wild guess, but (on the assumption that you endorse this quote) is the thought that MWI stands in relation to experimentally testable physics as something like a metaphysical thesis, and that instrumentalism doesn't lack metaphysical theses of this kind, but simply refuses to acknowledge and examine them?
Anyway, a related quote, and so far as I know the oldest of this kind:
The man who is ready to prove that metaphysical knowledge is wholly impossible..is a brother metaphysician with a rival theory of first principles. -F. H. Bradley "Appearance and Reality" 1893
Reminds me of Popper (World of Parmenides):
"The adherents of the Vienna Circle of logical positivism and of the Mach Association used to say that systems of metaphysics are merely the ghosts of departed scientific theories: scientific theories that have been abandoned."
Here's another installment of rationality quotes. The usual rules apply: