Referring to the list of French digraphs on Wikipedia, th is not present in the list, suggesting the French language does not seem to consider it as a digraph.

However, there is an existing page for the th digraph on French Wikipedia, listing its use in different languages, and stating:

This digraph is also sometimes used in other languages, such as French (in this case, the digraph comes from Ancient Greek), to represent the /t/ sound.

In fact, most of the French words featuring th seem to come from Ancient Greek:

• ἄνθρωπος (ánthropos): anthropologie
• ἐνθουσιασμός (enthousiasmós): enthousiasme
• μάθημα (máthēma): mathématique
• παθολογία (páthos): pathologie
• θερμός (thermós): thermomètre
• etc…