"Deliverance Lost" may give some clues about the way the Alpha Legion are able to replenish their losses faster than any other legion/chapter.
The Alpha Legion is the most powerful in matter of deception and planning, for sure. But I'm not sure that they are better than Iron Warriors for a siege, Word Bearers within the Eye of Terror or Ultramarines for purely battlefield tactics and strategy. Do you think, for instance, that they would have done better than the Thousand Sons against the Space Wolves, or than the Space Wolves against the Thousand Sons ?
The answer is yes. Clearly, all Legions are not equal and Space Wolves have no hope against full might of Alpha Legion even in the full frontal assault, and despite of their formidable psychic might, Thousand Sons are too small for making any difference.
Remember the fact that a splinter fleet of Alpha Legion was more than match entire might of the lauded Wolves and almost managed to obliterate them.
After reading the Calth, I started to doubt the vaunted battlefield adaptation of Ultramarines. Sever their heads, and Ultras become complete morons. Their doctrine is rigid and unimaginative, their tactics and strategies are time-proved hackneyed, their equipment are cheap and mass produced variants, their navies are meager and wholly dependent on the fleets of Excertus Imperialis, etc.
All in all, it is not surprising that staple tactics of 13th Legion is no other than mass assault using their number and discipline in the fullest...exactly same as the World Bearers.
Really, Forge World utterly shattered what rosy fantasy I had had about 30k Ultamarines.
In my opinion, Alpha Legions battlefield tactics and strategy, typically known as "Harrowing" is arguably superior than that of Ultramarines, if they are given enough time for preparation and before readied and complete their rehearsals, Alpha Legion would never pit against Ultramarines head-to-head. And anyway, they are much more flexible, pliable and creative than doctrine-bound Ultramarines. I bet only the White Scars can match Alpha Legion in terms of initiative, individual thought and creativity.
However they lack of Alphas discipline, force cohesion and unity of purpose and seldom cooperate -- simply put, they are too unpredictable and that is the reason why they never trusted by Imperial commanders, other Primarchs and indeed, Emperor himself.
And just one more talk. IIRC, you asked the source of the Alpha Legion recuperation speed. The source is Forge World Books and that is before timeperiod of Deliverance Lost. They always phenomenally fast at replenishing their loss and their gene seed was one of the most pure, stable and efficient from their incipient.
Yes, they are the closest thing of the Mary Sue in 30k galaxy.