Yes, a competitor can be determined to be an interested party. Please check ISO 9000:2015 definition of the interested party: “organization that can affect, be affected by, or perceive itself to be affected by a decision or activity”. Clearly, a competitor fits the definition.
Remember, ISO 9001:2015 uses the word determine. It’s a decision of the organization.
You can find more information below:
About context and interested parties you can see this free webinar on-demand - ISO 9001:2015 clause 4 - Context of the organization, interested parties, and scope - - where I present examples of interested parties and their requirements and expectations.