Whoever told you this was incorrect, because it depends completely on the specifics of the set up.
Z wave will be one network, zigbee will be a completely different network. They are both controlled by the smartthings hub, but otherwise they aren’t visible to each other.
A network that had 1 zwave light switch and 29 zwave battery operated sensors would be a much weaker setup than one that had 5 zwave light switches, 10 zwave battery operated sensors, Five zigbee plug-in modules, and 10 zigbee battery operated sensor’s.
It’s not just about how many devices you have. It’s about what kind of devices they are and where they are physically located relative to each other.
Read post 11 in the following FAQ, then go up to the top of that thread and read the rest of the thread, and it will explain about why the different types of devices matter and how you should lay out your network.
There are some things that zwave is better at and somethings that Zigbee is better at. So many people, myself included, chose SmartThings particularly because it can support both protocols, and that lets us match the best device to each individual use case.
http://thingsthataresmart.wiki/index.php?title=Z-wave_versus_Zigbee
There are also people who prefer to have all their devices be of the same protocol, and there’s nothing wrong with that. But you still have to put time and thought into thinking about where the specific devices go and just what kind you should get in order to keep your network strong. Choice is good. 