I got an answer to this elsewhere. The enabling or disabling of the Siri capability (and others) is actually stored in the project's Entitlements file. So I duplicated my existing Entitlements file, removed Siri from it, and set that Entitlements file as a variation for the macOS SDK in the project's "Code Signing Entitlements" build setting.
Topic:
App & System Services
SubTopic:
General
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