It is amplification if you allow the packets through transparently because all the hosts behind your firewall will respond if you send an echo request to the broadcast address.

So you're going to have to do a little bit more configuration than just allow a maximum packet size if you're going to allow ICMP to transit at all you should also limit the allowed set of addresses (you should do that regardless, but echo can be used for amplification requests by virtue of the broadcast feature of the IP protocol). Hence the 'one step away'.

This was known as the 'smurf' attack. Fortunately this is now mostly a thing of the past. But poking holes in your firewall for ICMP is a delicate affair.