Well, keep in mind that in Horus Heresy, the Lords of War are
Imperial Super Heavy Tank or Detachment (1-3 SH Tanks/Walkers of up to 3 Structure Points Each)
Single Titan
Single Imperial Super Heavy Flyer
A squadron of 1-3 Imperial Navy Flyers (imperial armor aeronautica)
1-3 Ordo Reductor War Engines
Primarch of the Legion used as the force's primary detachment
I don't think it's a good comparison to use the rules outlined for HH "Lords of War", and the 40k "Lords of War" because the rulesets are completely different. The average super heavy tank in HH is 350 points and get get ludicrous upgrades (like being immune to melta, or having a shield in the front that reduces the strength of shooting attacks by -2). Average super heavy tank for 40k LOW is around 500/600 and get none of those.
Primarchs run between 300-500 and depending on who you take, they are unstoppable death machines.
So the 25% limit fits well with what they have listed in HH. I don't see it working well for 40k.
"makes sense to me, even at the 2500pt level an opponent may not have the tools to take down a 900pt lord of war. "
No, at 2500, the max you can take for a LOW is 625 points. You would need to play a 3600 point game to get a 900 point LOW, and at that point, you might as well just be playing Apocalypse.