But there is a smallest unit, which is called a bit. Data can be broken down into smaller, countable units.

That’s not a particularly compelling argument. There’s a smallest unit of sand, too, but we still use a mass noun for it.

Besides, dictionary researchers agree it’s both a mass noun and a plural noun. People use it both ways. Here’s what Merriam Webster says about it. (I’m going to rework it to reduce the wordiness because it was so dense!)

Data leads a life of its own quite independent of datum, of which it was originally the plural. It occurs in two constructions:

1. as a plural noun (like earnings)
   • taking a plural verb and plural modifiers (such as these, many, a few), but not cardinal numbers
   • serving as a referent for plural pronouns (such as they, them)
2. as an abstract mass noun (like information)
   • taking a singular verb and singular modifiers (such as this, much, little)
   • being referred to by a singular pronoun (it).

Both constructions are standard. The plural construction is more common in print, evidently because the house style of several publishers mandates it.

So OP’s post is only half right, if even that much. In common speech, data is a mass noun, but many scientists and publishers still treat it as a plural noun. I would even venture most do.

Working as a programmer, most people I’ve interacted with use it as a mass noun, but not all. Language evolves, and the mass noun version is just as acceptable in most circles, but it certainly isn’t worthy of a “you should know” or “today I learned it’s actually a mass noun.”