The plural rule for words ending in “y” is to just add an “s” when the letter before the “y” is a vowel and to drop the “y” and use “ies” when the letter before the “y” is a consonant.

So:
Treasury -> Treasuries because of the “r” before the “y”.
Monkey -> Monkeys because of the “e” before the “y”

With that said, it may be that the WSJ has adopted the archaic spelling (e.g. “monies”) nonetheless.

I believe it is actually the “correct” way, but over the years, “Treasuries” has also become acceptable.

I still use “Treasurys” in my writings. Is the plural of “monkey” not “monkeys?” It certainly isn’t “monkies.”