Emm, I confirmed that one of my test platform (esp32c3), its cdcacm driver implement the uart_ops, but rp2040 seems not. CLE should works since it will override the content from driver ECHO if over serial driver, but readline shouldn't echo input. I'll test more to confirm it.
cdcacm.c is the part that is similar to serial.c. It implements the serial driver interface and also interfaces with the lower level USB device driver. cdcacm.c resides at drivers/usbdev/cdcacm.c and it common for all MCUs. The MCU-specif logic has no idea that there is a cdcacm "gadget" attached to it.
cdcacm.c implements the common uart_ops that are used by all MCUs to support CDC/ACM serial.
pl2303.c is another USB serial device. It also implements uart_ops and has its own TERMIOS implementation.
