It is a script path spend.
Key path spends are defined by only have a single witness stack item which is interpreted as a signature. If that stack item does not validate when interpreted as a signature, then the spend is invalid which renders the entire transaction invalid.
Script path spends are defined by having more than one witness stack item. This is always true as script path spends must provide the control block and the leaf script, and these are separate stack items.
In order for a leaf script to be revealed, it can only be in a script path spend and all script path spend rules must be satisfied.
Inscriptions do actually utilize a leaf script which performs an OP_CHECKSIG. The construction is
OP_CHECKSIG OP_0 OP_IF OP_ENDIF
