Zelensky is not a first-hand witness to the charge. The "phone call" is the only direct communication between the two of them. Zelensky himself would have mostly third hand information. He would have heard the specifics of the demand via his aids, who would have heard it via Sondland/Volker/Rudy.
If Zelenskey's aids completely shielded him from all the backchannelling, that would in no way impact what's Trump's intentions were.
Sondland is really the only person in communication with Trump that we've heard from. Absent we hear from Mulvaney/Rudy, we're forced with connecting the circumstantial dots.
So by the time it was all said and done, Sondland, Volker, and Giuliani all said that none of them communicated to Zelensky or his aides that trump was withholding aid until an announcement was made. They were the only people with direct communication with Zelenskys advisors.