No, you've got a destruction going on. Consider:

• process 1 opens for create - file deleted
• process 1 flocks - and continues
• process 1 writes its data
• process 2 opens for create - blam, process 1 data is gone
• process 1 closes, releasing the flock
• process 2 flocks - and continues
• process 2 writes its data
• process 2 closes

I guess if your goal is to have only the most recent data, you've succeeded, but you didn't need to do the flock for that... you can just leave the flocks entirely out.