If this IS a source for GW's inspiration, they are indeed playing a very long game.
The star Alphard is classified as a Barium Star, which is to say that after it's "brother" (or twin) died, Alphard became "contaminated" or took on heavy metallic elements of the dead brother (Praetorian of Dorn's ending and Omegon taking up Alpharius mantle much?). Because of Alphard's contamination, it too will die off soon (Guilliman killing "Alpharius?"). Fucking grimdark.
Some of the speculation about the AL goals and the true extent of their forces is a bit cringey, like how they'd turn on the traitors during the 2nd siege of Terra since they've built their forces constantly for 10K+ years, and the Emperor would laugh maniacally at Chaos...
Basically the things that follow "if one thinks about it" and similar lines, tend less towards interpreting canon and more towards fan theories and power-fantasies, which, fine, some of them are entertaining, but do they really belong here?
I'm not a fan of the section where it say AL trained their entire Legion to match the elite specialists of other Legions in EVERYTHING, either. Even if EVERY marine was trained in EVERYTHING, they'd have natural predispositions towards certain skillsets, and a AL tactical marine spending a few battles as a heavy support guy wouldn't be able to match an IW iron havoc, for example - the AL would lack the experience, and this is assuming the AL actually had the gear and munitions to field and maintain that number of specialist equipment. A more restrained take would be that AL were required to maintain a basic competency, so that in case of sudden losses they could still field specialists and vehicles.
Alpha Legion affiliates or agents?
YouTuber Remleiz put out a theory that the Ultramarine successor chapter, The Sons of Orar, could possibly come from Omegon's geneseed. It's inconclusive, but it would not be the first time Gulliman made loyalist members of a traitor legion honorary Ultramarines (see Barabas Dantioch, formerly of the Iron Warriors). So perhaps this "Orar" came from the Alpha Legion? Thoughts?
Another organization that has a curious naming scheme is the Militarum Tempestus regiment known as the 55th Alphic Hydras. The Sixth Edition of the Militarum Tempestus Codex mentions that the regiment has fought alongside, trained with, and had younger members recruited into the Ultramarines. Their specialty includes Shock Assault, Targeted Decimation, and Planetary Interdiction. This "highly respected" regiment's badge contains more than one meaning: (1) it refers to a sigil from the Dark Age of Technology that has been linked to the regiment since their inception and (2) is a reference to Mount Charas, where their training facilities are supposedly located. The Codex doesn't state what planet Mount Charas is on nor who is the commander of the regiment. The Alpha Legion is known to raise up cultists and human operatives to infiltrate organizations, so it's not much of a stretch to say that this regiment could be a target for the Alpha Legion (the fact they are called the Alphic Hydras should be a delicious irony for the Alpha Legion not to pass up). It would be interesting to see if this comes into play if more lore comes out.