I believe it could be true and here is why.
In 3g land voice and data are on seperate connections. In 4g it is all IP so voice and data travel on the same pathway- voice and data are all packets on the network. This type of architecture is how most business T1s are provisioned these days. Provider delivers a data T1 (I am not using the word Internet very intentionally here) and it is a private connection to their network. The carriers core network then determines if the packets are voice or Internet and routes them to the softswitch in the case of voice- or the public Internet and the case of Internet.
The most important element to ensure good voice quality in network like the above (VoIP network) is the ability to identify what packets are voice (and need special prioirty) and which packets are data and don't. Multi prototocol label switching (MPLS) is what is used to do this. Sprint runs a pure all MPLS backbone so they can queue traffic how ever they want.
So- when you are talking on the Evo your packets will absolutely have priority over other users data (including other Evos surfing the web, youtube, etc) . The question really becomes wether Sprint will prioritize "standard" data coming from the Evo over other 3 g devices. They certainly could do it- not sure if they actually will.