There is not team that is 100% bad so you don't need a full rewrite of 100% of the code base.
What people rewrite is not really bad code, it's just middling annoying muck code.
The first kind was written by people who know about software, has some attempt at following some conventions, variable names, etc. etc. It's not "clean and shiny", but you can see someone tried. Like if a bunch of first year apprentices built a house. They tried their best, and often did a half reasonable job - at least for a first timer. Plenty of companies are using this kind of software to make many of millions of dollars. I would say this is extremely common in the real world.
And then there's codebases that consist of a single source file that has hundreds of thousands of LOC and that file is called indexNEW.php, indexNEWNEW.php index2022-1.php (and so on). Nobody who works on it has ever studied any kind of software course at any kind of school, and literally nobody has any real understanding of what/why/how, because it gets passed along like a hot potato everytime someone quits due to the insanity. Nobody who works on it even knows what source control is, has no idea why you shouldn't copy and paste code, and has never heard of TDD or automated testing or bullding or literally any software discipline. This isn't like first year apprentices building a house, this is like Homer building the Canyonero - an utter disgrace that just barely functions at all. [1]
I wouldn't be surprized if Joel is talking about type one, and doesn't even really consider type two to be "software" or "a system". It's just a stinking pile of garbage.
(I worked at a very large Telco. The vast majority of code plugging the systems together was type two - it was a nightmare of spaghetti so bad that when a power outage and failed backup generator took down everything (including 911), it took more than two weeks just figure out how to start everything back up again.)
Imagine a file with 100,000 LOC, some commented out, copies of functions like "addCustomer()" and then "addCustomerNEW()" and then NEWAddCustomer() and then "makeCustomerDanGNewest()" and so on and so forth. Absolute gong show.