It really depends on what you consider "making a living" or "could support themselves". In the UK the mean income for all full time writers of books - pulled up massively by the big successes - was below minimum wage last time I saw a proper survey. The top 10% pulls in about half the total income for that group in the UK.

Most of the full time writers of books would not be able to afford being full time if not for partners (their average household income is above the UK average), and/or supplementing with other income, such as writing articles or doing additional work copy-editing/proofreading etc.

So while it may be a bit hyperbolic to say it's as rare as being a billionaire, the profession is also "subsidised" heavily by partners etc. making it possible for people to hold a low paying job that they wouldn't hold otherwise, and the number of people who earn a wage they'd be willing to live on without higher paying partners, is absolutely tiny.

I've self-published two novels and working on the third and they've sold considerably better than average yet couldn't cover what I spent on takeaway last year (to be fair, I spent an exorbitant amount on takeaway; the kind of money people on minimum wage would be aghast at, and I'm being better this year, but the point is I could without even noticing because it was a rounding error). Your co-worker is a runaway success. And that's amazing, and he should be proud. It's also an extreme outlier.

But to go back to start, it's also subjective and down to what you consider enough that it could "support you".

Put another way: Even if I had a NY Times bestseller, if it was "only" towards the lower end of the chart, I'd lose my house if that was my only income. I can't afford to be "just" a NY Times bestselling author unless it's far up the chart, even if I could write well enough (if I could have multiple books on the lower end at the same time, maybe, but most authors don't write that fast)

And the vast majority of authors are nowhere near selling at that level. I know or have a good idea of the income of several full time authors who regularly are on the bestseller lists that earn incomes that, sure, technically could support me, but that is way below what they could earn in other jobs given their skills. I work in tech, and I know the earnings of one very successful UK author with a past working in tech, with a multitude of awards, 20+ years of published backlog, who still earn less than me. That's not a "haha" - I'd love to be in his position and feel I could afford to write full time, and it's amazing to have stuck it out to get to that level.

Now consider that most authors don't write more than 1-3 books in their lifetime. Some because it's all they want. Most because they don't get sales. "Any" sales.

You may have noticed I've added the qualifier "of books". I did that for a reason: If you want to make a living writing, journalism (still badly paid), magazines, or copywriting etc. are much better paid. If we include that, then the claim is definitely hyperbole. They're still on average not well paid, but the average person who writes words for a living in the UK are pretty much earning an average salary. Most would still make, on the basis that a well-earning author is overall going to be quite smart and well educated, make more in most other jobs they'd be suitable for.