This 'accept' thing confused me as well.
As far as I understood when considering this option myself, the only way to 'accept' the interest letter is to file your application directly to the OINP. After that they will consider you for nomination. If they do nominate you - only then you can actually accept or decline this nomination.
3. Apply to OINP under the Human Capital Priorities Stream
After you receive a PT Notification of Interest from Ontario through your MyCIC account, you have 45 days to apply to OINP under the Human Capital Priorities Stream.
To apply, fill out the Nominee Application Form (see Section 4 for instructions on completing the form) and ensure that you have all the required supporting documents (see Section 3).
All applicants must indicate which federal immigration program they wish to be assessed against: FSWP or CEC.
OINP will assess your nomination according to the Human Capital Priorities Stream eligibility criteria. Your application will also be assessed to ensure you meet the FSWP or CEC criteria.
4. Accept nomination from Ontario and apply for permanent residency
If your application is approved by Ontario, you will receive a letter from OINP through your MyCIC account notifying you of your nomination. You have 30 days to accept the nomination from Ontario in the Express Entry system.
Nomination from Ontario will give you an additional 600 CRS points and you will receive an invitation to apply for permanent residency from CIC.
Once you receive an invitation to apply from CIC, you have 60 days to apply to CIC for permanent residency.
During the assessment of applications for permanent residency, CIC reviews applicants for admissibility, including health, security, and criminality checks as per Canada’s Immigration and Refugee Protection Act. CIC will also verify all information provided by applicants to register for Express Entry.
