Before forming any kind of opinion, I think it's necessary to read the full context. There's a distinction made between a formal / solemn oath and a simple proposal or proposition. Nowhere does it say that anything is or is not a sin, is or is not "OK", just questioning whether something is a true "oath" vs. a simple statement.
So, for instance, there's a textbook case of someone who perhaps promised virginity to Our Lord but then changed her mind later and wanted to be married. This type of promise is not considered a vow or oath of any kind and can be dispensed by a Confessor.
I know that we often have this type of scenario with our kids. We might say something like, "We'll go out for ice cream later." But then things happen and we change our mind, and the kids would immediately chime in that we "promised" to go for ice cream. We did no such thing, but were merely proposing something at the time, without any intention of binding ourselves to it. That's precisely what this (out of context) passage reminds me of when it refers to a "simple proposition" as opposed to some kind of "promise".
But NOWHERE here does St. Alphonsus say that "perjury is OK". He's merely discussing whether a certain proposition or statement takes on the nature of a vow if one doesn't intend to bind oneself to it ... just like my ice cream story above.
In fact, I'd bet that St. Alphonsus would say it's a sin to lie when under some formal oath, as in a court proceeding, and would hold that such is a true vow or juramentum vs. a simple proposition.