- As a shout-out and homage to Dead Rising.
This is why Sam B. and his fellow immune from the prior outbreaks aren't true Numen, they came close while escaping from Henderson in the boat with the other survivors but avoided attacking them somehow, most likely by barricading them in the boat's cabin while they went through the same thing the Slayer did in the lab before Tisha arrived. This is the incident that broke the group up as they realized that by not attacking each other that they had a deeper connection through the virus (or what they thought was a virus) and if they stayed together they could potentially cause another outbreak. Any other potential Numen were killed without them realizing their potential, possibly by Sam and the others if the below WMG is accurate.
As seen in the Mission: Rainier post game quest, any zombie above Walker status was a Numen potential that didn't make the cut, so to speak. This also applies to the Elite Zombie variants from the previous games as the Slayers' skills reflect many of the abilities the Apex use against the player. The screen flashing when the player meets the first Apex boss is heavily hinted to be the Autophage hivemind trying to take control of the Slayer, plus Moose and Dillon show recognition of Alex after undeath and even hang out at spots they frequented while alive. If Numen are Autophages that still have self-control and free-will they are technically Apex variants, see Thurston and Cadenza's resemblance to Crushers and Screamers respectively (and confirmed by Word of God to be intentional) and the fact that Slayers can use the main Apex attacks while in Fury Mode.
Or even the main group behind both of them, tying into the rest of the series' shadowy organization hinted in the lore. What information we have of Eschaton seems to indicate that they're interested in Numens specifically and knew about the Autophage years before the Banoi outbreak if Reuben Reed is to be believed and the Cosplay costume options for the Slayers heavily imply that Eschaton identify potential Numens in the population.
The Artistic License – Physics displayed by modded weapons and the environment throughout the franchise is actually how they work In-Universe despite the mostly realistic setting. In turn, technology is more advanced because of research into these powers which is why a battery can provided a charge to a machete that can electrocute a person and med kits can help seal a torso wound that would otherwise require surgery.
- Semi-confirmed in the Haus DLC for the Autophage's ability to give zombies superpowers and revive them in the first place is because of a omnipresent energy field that powers all living things, Konstantin's research creating crossbows bolts that make zombies' heads explode and emitters that can recharge the Slayers' Fury Mode.
Sam being in the main game made sense because of his connections in the entertainment industry and the first expansion Haus involves a billionaire that might have connections to Lola or Eschaton. Xian Mei is, as seen in Escape Dead Island, no stranger to dropping into zombie-infested locales to extract someone and gain valuable intel, so taking a trip into HELL-A is right up her alley, though the Slayer will warn her that like Sam she's not entirely immune to the new strain of HK once they realize who she is. Xian will start out as a Defrosting Ice Queen as she works with the Slayer and will reveal more information in a quid-pro-quo arrangement, leading to reveals on both sides.
- Jossed. Xian doesn't appear in either DLC.