There are more kind of letters then just our latin alphabet. Cyrillic, aboriginal Canadian and even Chinese and Japanese can be typed by just text for instance (日本語の例え)
In the same vein, mathematical and other scientific symbols can be typed using just text, and 𝕏 is the symbol that stands in place for arbitrary distance in a formula.
Basically, it's just a (mathematical) letter, it can be typed.
That's also why their new CEO had difficulties copyrighting it; it's just a letter, after all!