In the Local and Push Notification Programming Guide Apple states
Important: In iOS, Wi-Fi is used for push notifications only if there is no cellular connection or if the device is an iPod touch. For some devices to receive notifications via Wi-Fi, the device’s display must be on (that is, it cannot be sleeping) or it must be plugged in. The iPad, on the other hand, remains associated with the Wi-Fi access point while asleep, thus permitting the delivery of push notifications. The Wi-Fi radio wakes the host processor for any incoming traffic.
Sending notifications too frequently negatively impacts the device’s battery life—devices must access the network to receive notifications.
What is meant with "for some devices"? Are that iPods and iPhones without cellular connection?
If I get this correctly always the cellular connection is used. If not available Wi-Fi is used under certain circumstances (as stated above).
"For some devices" means that the rule applies sometimes but not always. Apple isn't intendng to list devices for you in dogwhistle and isn't binding itself to any particular behaviour now or in the future. The words have no meaning other than their natural meaning. Though the examples you posit sound like two likely examples.
Otherwise your interpretation seems correct. It seems to be: use the mobile network if available; otherwise use Wifi if available. Some sleeping devices may have neither.