You can often easily oversubscribe any port if there are multiple ingress ports sending data to it. I.e. you don't always need to have a larger bandwidth link send data to a smaller bandwidth link.
"Line rate" just means the port can sustain the full rate, indefinitely. Usually billed as a feature when all the device's ports can do so, concurrently.
If the oversubscription is temporary (i.e. short duration), that's where buffering comes into play. The larger your buffer resources, the more data you can buffer before you need to drop any. (As buffer memory, generally, needs to be "high speed", it's expensive, which is why devices have limited amounts of it.)
