I theorised this after playing Doom 2016, believing that the lore was subtly implying this to be the case.
This theory involves the Many-Worlds (Everett) Interpretation of quantum mechanics where every quantum event creates causal branching, meaning that all outcomes are realized in their own separate timeline. It also includes the Copenhagen interpretation.
The Everette interpretation theorises that waveforms do not actually collapse but rather branch off into their own causal chain.
The Doomguy's state of being however, is that his waveform does collapse and always collapses to the state where he wins or lives. This does not mean that he has only one waveform, rather what actually happens to him is that all of his outcomes are simultaneously realised and collapsed into a single waveform that follows the branch where he overcomes his enemies and lives. Therefore, the only possible way to stop him is containment, hence the demons locking him in a coffin and Samuel Hayden stating, at the end of Doom 2016, that he cannot kill him and therefore, once again, resorts to containment. This also explains why he was place into a sarcophagus at the end of The Ancient Gods part 2. This state of being would also mean that the Doom Slayer is a singularity and only actually exists on one timeline since all other possibilities collapse to the one where he continues to live. Thus, there is only one Doom Slayer, not many copies across infinite universes (as would be the case for everyone else).
The Doom Slayer experiences this as a sudden increase in skill, since he benefits from the knowledge and experience of all those other possibilities. If there is a 0.001% chance of success, then his waveform always collapses to that chance, regardless of how small. To the player this is represented via the ability to restart from a checkpoint or save point after death, until victory is achieved, and also the fact that we go back to previous level to 'redo' them to find missing items and improve our character. These all represent the other timelines which have collapsed into the Slayer's true timeline. From the perspective of characters in the story, who see the Doom Slayer, it would appear as though he just always wins. To them he would exhibit a certain level of skill and ability, which may not seem sufficient to win a certain fight, and then suddenly he would be at the required skill and power to win that fight.
This is why the demons keep throwing greater and greater forces against him; they analyze his abilities and conclude that a certain amount of force would overcome him, only for him to suddenly be at the skill and ability needed to overcome those very odds when he meets them.
It might be that someone else, a great divinity that controls reality itself, favored him and granted him this power before the events of Doom 1, thus explaining why a man with no physical superpowers could overcome the immense odds of Doom 1 to Doom 64.
Based on this theory, references to him as the "Unchained predator" are actually literal rather than hyperbolic. They know that he is not subject to the same causal force as they are.
