That’s an explanation for the origin of the word, but as far as I can tell it’s not a standard spelling in modern Japanese. At least I don’t see it referenced in dictionaries.
It’s also a bit weird since it obfuscates the fact that くノ一 represents the strokes of 女 which is the whole point.
I suspect that the author of the sentence got a bit zealous with kanji usage, given that this is a kanji learning platform and all that.