What you’re saying all makes sense, and most sellers who have been here a while are very clear on Fiverr’s TOS.
However… Fiverr have been known to be a touch gung-ho when it comes to enforcing their terms against sellers, sometimes in a contradictory manner. We’ve all read posts by sellers who have had their accounts shut down with little warning for apparently minor infractions. And I’m afraid the rule that Fiverr insist we stick to most rigidly is not taking business away from Fiverr. If Fiverr CS get so much of a hint that you might be attempting to transact (now or in the future) away from Fiverr, you’re liable as a seller to lose your account. No second chances, your business is destroyed.
I know this wasn’t what you were attempting to do here by the way. I totally get that the email was an administrative necessity for your project. But by having your seller’s email address, it could be argued that you could avoid Fiverr in the future, and deal directly with the seller. There is a chance that Fiverr CS would interpret this as the seller attempting to avoid the 20% fee by getting your contact details to take business away from the site. Most sellers, ourselves included, simply will not take that chance, regardless of what the TOS say.
Because of this, your seller absolutely did the right thing. We would have (and have done) the same. They COULD have contacted CS on your behalf to request written permission, but honestly, unless you were offering them a lot of money for the project, most sellers would see this as a lot of extra effort, and still a risk.
Sorry - I know that doesn’t help your situation. But as a rule of thumb, regardless of what the TOS say, if you can avoid sharing contact details in any way possible, you’re more likely to have a successful transaction with your sellers.