The Voynich Manuscript is a cipher because...?

[Emma May Smith]

Of your three points in favour of a cipher, I agree that point 2) is good. But point 1) is ambiguous, and point 3) is incredibly thin.

Point 3) is not that thin when we begin to consider possibilities of what this label may stand for. Every possibility yields quite strange outcome. Like, if it stands for a number, then from the viewpoint of an unknown language we should admit not only unknown script, but also an unknown numbering system. If it is a noun, then we have a one-letter  noun (is that at all possible in a non-hyeroglyphic script?) And so on.

Many scripts at this time used characters to represent numbers. Being both an unknown script and an unknown numeral system would be pretty much normal.

Many languages have nouns which have a single sound.

Most importantly, it's not hugely convincing that this single stroke is a label.