Advent players build mostly disciples early on because illuminators are tier 3, expensive to build, and not very good compared to kanraks and LRMs...Advent has both the best LF and the worst alternative options for early fleet composition...that doesn't make LFs good, it just makes them the least crappiest strategy for Advent early on...
Later on, it depends on the player and the situation...I have not played legolas, and I don't mean to downplay his achievements, but if he has only disciples then his fleets should be pretty easy to beat with LRMs or kanraks...even HCs would crush a horde of disciples...if he is seriously owning people with just disciples then either his opponents don't know good fleet composition or legolas is a extraordinary player in other ways (like time management, being proactive, microing, etc...)...in either case, that says nothing about LFs...
I have played Advent quite a bit and have had my own hordes of disciples (somewhere generally in the 50s or 60s) but I never relied entirely on LFs because it simply does not work...a good player will not spam support ships, flak, and carriers without having the LRFs or HCs to protect them...
I play with a mod that makes LFs far better than they are now, and even with those changes LF heavy fleets are very easy to destroy with HCs and LRFs...the only reason an LF heavy fleet would succeed is if you heavily outnumber the enemy or they went heavy flak/support/carriers to preemptively counter an LRF heavy fleet...in either case, it wasn't LFs that gave you victory, it was your opponent's poor strategy...
I by no means am claiming that LFs should never be built...what I am getting at here is that they make up a very small portion of most fleets...in the cases where a fleet is almost entirely LFs, we are either looking at an Advent player who had few other options or a situation where the enemy fleet was heavily unbalanced...