BGP main loop prevention mechanism

The only usable metric do prevent loops in bgp is the as-path attribute. No EBGP peering neighbors do accepct an update which includes it's own AS. So this is used in EBGP to prevent loops. But in IBGP there is no change is this attribute, so there comes the rule you mention with the IBGP full mesh. Of course you can overcome this limitation by either using Route-Reflectors (which add two additional attributes to prevent loops, CLUSTER_LIST and ORIGINATOR_ID) or by using Confederations which splits your AS into multiple SUB-AS's which use EBGP to connect each other.