Better chance at a F upgrade by forgoing C+ upgrade?

Hi all,

I just started a Platinum status match challenge with Delta. I'm currently on my first flight for the challenge (DL 555 SFO-JFK) and despite around 16 seats available at T-24, I was not upgraded to D1. I was however upgraded into Comfort +.

A DL loyalist friend of mine suggested to me that when booking a ticket in the main cabin, it may be harder to get a "double upgrade" (i.e. Main to C+, and then C+ to F or D1) than to opt out of the C+ upgrade and just request a direct line from main to F/D1 (if available).

Has anyone else found this to be the case? And if so - is it the same whether you upgrade yourself right after booking as a Platinum Medallion, or whether you are upgraded by the system?

Would appreciate any insight as this is definitely not the case on AA!

Thanks,
Daniel