half duplex for Atmel SAMD21
Up to six Serial Communication Interfaces (SERCOM), each configurable
to operate as either:

USART with full-duplex and single-wire half-duplex configuration
I2C up to 3.4 MHz
SPI
LIN slave


It is an error in the overview of the SAM D21 features in the datasheet. The SERCOM is half-duplex for I2C. USART can't be configured half-duplex.
see Application Note about SERCOM configuration

1 Introduction to Serial Communication Interfaces (USART, I2C, and
SPI)
... The exchange of data can be half-duplex or fullduplex depending on
the serial module specification. ...
1.1 USART
... It is full-duplex in operation. ...
1.2 I2C
... It is half-duplex in communication. ...