As explain in the Horus heresy serie custodes are the ultimate warrior (save for emperor and sons) but they are not soldiers.
They can outmatched any Space Marine on a 1 on 1 basis, but can't fight thousand astartes even if their number is the same, they never learn to fight as an army, that was never their aim or their goal. They was and still be the bodyguard of the emperor, the blade parrying the assassin blade. The shadow in the shadow. Why do you think they wear such armor, because all who don't are invisible and people aren't looking for Custotdes not wearing armor of gold.
By the way 100 astartes aren't commun response to massive waagh, maybe a medium size one , but look at armagedon and tell me how many astartesq and IG soldiers was send their and how long the battles take plaec… Second and third war.
Same 100 astartes are what is needed to put down Angrom alone and when I say astartes you need top hear grey knight.
It is true that most of the time a SM compagny is a force able to take planet. But as the Crimson Fist have learned sometimes a full chapter isn't quite enough…