Sounds like BS. Even if you almost never use them, brake fluid has a finite life and needs to be replaced every few years. Brake systems are never perfectly hermetically sealed and brake fluid is hygroscopic.
Rust is usually rubbed off when the brakes are engaged. Buildup on any vehicle is only really an issue when the vehicle sits undriven for long periods of time.
In EVs the brakes are engaged, just less frequently and with less force. For example in a normal car when you depress the brake pedal gently to slow to a normal stop the brakes engage immediately. In an EV the regenerative braking force increases when you depress the brake pedal and only after it has hit 100% do the brakes engage.
So with an ICE vehicle the brakes are the primary stopping force most of the time where as with an EV they're the secondary stopping force most of the time.
I think he may be referring to rust inside the brake lines. I've seen it before on an older vehicle that probably never had its brake fluid changed. Over time, moisture gets into the system and corrodes the steel brake lines, causing rust to get into the fluid. It's why you have to change the fluid every so often: it's impossible to keep moisture out, and the fluid is hygroscopic.
Please stop getting involved in flamewars about brakes or otherwise. Tedious back and forths like https://news.ycombinator.com/item?id=19001513 are something we're trying to avoid here.