It wasn't specifically in the context of debug pins.

On a "normal" arduino, an FTDI chip on the board handles the job of exposing a serial adapter to your computer over USB. The atmel chip on the other side of the FTDI chip runs your code and getting serial out from your firmware is a short codepath which directly uses the UART peripheral.

On a teensy, there is still a secondary chip, but its just a small microcontroller running PJRC code. This microcontroller talks over the debug pins of the main chip, and those pins aren't broken out (at least back when I last used a teensy). Despite covering the debug pins, this chip only handles flashing and offers no other functionality. Since there is no USB serial adapter, for hobbyists trying to use it for running code with an arduino HAL, the HAL has to ship an entire USB driver just for you to get serial over USB. And this itself means you can't use the USB for other purposes.

For advanced users, this makes debugging much harder, and god forbid you need to debug your USB driver.

It's kind of just a bunch of weird tradeoffs which maybe don't matter too much if you are just trying to run arduino sketches on it but it was annoying for me when I was trying to develop bare metal firmware for it in C.