Are the discrete I/O considered to be part of the ARINC interface?
It depends on which ARINC document you're talking about. ARINC 429 is a digital serial bus standard. Discrete data may be sent in ARINC 429 data words, but that's just bits in a word not the physical discretes you're asking about.  

ARINC also has documents that cover standards for other interfaces as well as 'Characteristics' that define specific LRUs such as ARINC 735A for TCAS II.  In these documents ARINC does specify discrete interfaces.  There are two types, open/ground and 28V/open and are defined for both input and output.  

So yes, there are discrete interfaces that are part of ARINC standards, just not part of ARINC 429.