>"The Xbox was a powerful machine, offering highly detailed graphical capabilities that would be on par with low end PC machines at the time"

The Xbox's NV2A could trade blows with a GeForce 3 Ti500, which was considered by many to be the best-performing consumer graphics card available in late 2001. It's not like people were building their PCs up with crazy multi-GPU setups either; 3dfx SLI was a thing of the past, and Nvidia SLI and AMD Crossfire wouldn't exist for a few more years. At the moment of its launch, the Xbox's graphical capabilities were competitive with good PCs.

>"It also advanced Bungie’s abilities in pushing for new gameplay ideas; Marathon was the first major FPS title to contain a backstory, told through computer terminals throughout the game."

Not really. Story integration wasn't a new thing for Bungie, and of course you have things like System Shock existing.

>"This paved the way for what we have today, simultaneous releases for PC and consoles, often the same game either merely ported or barely altered to now encompass both console and PC users. Halo was one of the first titles to showcase that crossover appeal, especially after the PC release of Halo in 2003."

Kicking off port projects post-launch was already a common practice for successful games at the time, and Halo wasn't even particularly expedient about it. Variants of Doom existed on several consoles within a year of its PC launch, for instance.