Skip-level thinks O3s are a waste of time. Suggestions?

When I stepped in to lead a software development group, I initiated O3s with my directs and asked that they perform O3s with their directs for a trial period. 

I have received positive feedback from many on the team, but one of my skip-level, senior software engineers complained that the O3s with his manager (my direct) are a waste of time.  It's worth noting that he's matrixed to project manager outside of my group.

The engineer cited the following complaints for weekly O3s (preferring occasional, unscheduled discussions):

• "My functional manager is only here to sign forms, such as vacation requests."
• "I provide status to my project manager.  Why does my functional manager need to know status too? A project should have only one customer (project manager, not the  functional manager too)."
• "The relationship with my functional manager is fine.  I don't need to improve it."
• "Maybe O3s are useful for junior engineers, but as a senior engineer, I don't need any help or guidance from my functional manager."
• "O3s are a waste of time. A half hour a week for a whole year means three full days in extra meetings."

Do you have suggestions of how to make the O3s more effective for employees who think that they're a waste of time?