Those are just plain wrong. It's like talking about "English alphabets". There's only one. What they mean is "hiragana characters", etc.
It looks like it’s been edited and is far more agreeable to the idea that ‘hiragana’ et al are already plural.
It's not plural at all; it doesn't even make sense. This is like debating whether the word "happiness" is singular or plural.
There's no such thing as "a hiragana".
In regular usage there definitely is, it’s easy to find something like this on a Japanese website: “どの漢字がどのひらがなになったの？”
That means “Which kanji became which hiragana?” If there’s no such thing as ‘a hiragana’ that wouldn’t make sense.
As for kanji, “この漢字” also shows up all the time when referring to a specific character. References to specific amount of kanji are everywhere too, eg the “100 Kanji” here. https://www.kinokuniya.co.jp/f/dsg-08-EK-1085796
>That means “Which kanji became which hiragana?”
No, it doesn't. It means "Which kanji characters became which hiragana characters".
Words do not directly translate between languages the way you think.
Maybe a refactor is in order.
However, in my opinion, an article purporting to inform about Japanese in a programmatic way really should avoid using “hiraganas,” “kanjis” and “katakanas” as pluralization of thise terms.