As noted in the article, it could be a very sharp transition from "largest factorization performed so far is 3x7=21" to "we can factor real-world RSA."
If you want to make a classical computing analogy, it's like we're struggling to make transistors with more than a single 9 of reliability, and obviously you can't do complex computation with a circuit where every step gives garbage 1-in-10 times.
Except it's not's obvious. 90% reliability could probably be made to work with silicon transistors with bog standard error correcting logic at the hardware level. Quantum error is a little bit more troublesome to work with, but there are also no known theoretical reasons error correction wouldn't work at existing error rates. We just need better algorithms, which might very well exist.
Or, the next generation of chips would offer more 9's reliability, and even with existing error correction just a few more sigma in reliability would put us over the tipping point of being able to make reliable large-scale systems.
There were mechanical computers before the 20th century that had more complexity (in terms of total information) and were more useful than quantum computers are.
Not true. Programmable looms and player pianos existed. They weren't Turing machines, but they were certainly more sophisticated than mere calculators. And of course there's the analytical engine, even if it was never built. These technologies were far more influential (both culturally and within the context of CS itself) and practical than QCs are. It's possible if electricity had taken a bit longer to develop that we would've seen honest-to-goodness mechanical Turing machines during the early 20th century. It's not like just didn't know how to design and build the machines, they were just very complex and expensive.
So if you want to make analogies between quantum and classical computers, QCs aren't even at the level of early 19th century technology. They're still trying to figure out what's even necessary to build devices of a practical size.
I only wonder what it means for cryptography.
The letters "crypt" don't appear in the text.