(I worked at a video delivery startup a few years back and these were exactly the type of discounts we would get)
Also what percentage of their total customers are not paying list price? Is it more than 10%, 20%, 30%? I know that me as an individual I will be paying list prices.
And the problem with not comparing list prices is that of course you can have two or more companies getting discounts with the exact same bandwidth who will be paying vastly different rates.
In a video company doing 1 000 000 TB of transfer per month, you gotta negotiate with every provider for bandwidth, that's just the way it is.
Just like if you were working for a plane company, you wouldn't be buying fuel from the station down the street.
OTOH, if you are getting 90% off the list on both, you're talking $100M vs $10M. That's enough where features, SLA, QoS, matters more ($90M).
Saving a little might not be worth the labor costs and risk differences between two solutions. Paying a bit more to contract out a class of problem could be the thing to do.
Though probably not in Zoom's particular case. If this were Patagonia, or Home Depot, sure. Pay someone else's IT nerds to deal with the problem. But Zoom is IT through and through. They better understand the low level details of how their stuff works, or when they get into trouble they won't even see it coming, and we will wonder very loudly why not.