where the writer clearly got a whiff of plural formation in Italian, just not enough of a whiff to form the plural of feminine nouns and adjectives.
I don't bring this up to be fastidious, which I can be. I bring it up because the writer is clearly shunning the rules of plural formation with which both he and his audience are familiar in favor of ostensibly more scholarly and therefore more impressive forms. If you want to brandish your scholarship, then go ahead (we all like to at one point or another), but first make sure that you possess it. In the meantime, plain old English plural formation1 will do just fine. There's nothing shameful about curriculums or panna cottas or forums or octopuses. And sure enough, there's nothing shameful about rectums.
Incidentally, I visited St. John restaurant when I was last in London, but I did not order any recta. The chitterlings, however, were delicious.
1But not plain Old English plural formation because that would only make matters worse.