No, YAAM Ao100 does not give guaranteed DS drop.. it was just your luck.
Guaranteed Dragonslayer drop
-> Play standard game on Nightmare! difficulty, achieve killrate 100% before entering Unholy Cathedral, Clear Unholy Cathedral damageless
(Of course, even after spawning DS, wielding it is another problem)
Guaranteed Berserker Armor drop
-> Play standard game on Nightmare! difficulty, achieve killrate 100% before entering Mortuary/Limbo, wield Dragonslayer by clearing UC w/o damage, clear Mortuary/Limbo damageless
Also, from after wielding DS+BA, nightmare demons will be continuously spawned from everywhere you go (which blocks you from getting 100% killrate)
And if you nuke spider mastermind and survive from the nuke (by lava elemental or inv globe), Apostle will appear instead of John Carmack. You can get Apostle Insignia medal as award for killing it.
And VBFG assembly to nuclear BFG is not a good idea in my opinion.. it's still too weak.
For nuclear BFG, biggest fucking gun assembly makes it extremely useful, but without that, it's not so reliable weapon for most occasions.