That's not to say that the The McMaster site is in any way bad. It is visually simple, relatively free from distractions, and straightforward, and served its target demographic well. It frustrates me to no end when marketers (not designers, generally) insist on shoving crap in people's faces because metrics show the hard sell works.
That said, their design was made for a specific type of shopper and is not remotely generalizable to everyone else.
People used to looking at screens of code all day, and those who know e-commerce platforms inside and out might not miss the abject lack of visual hierarchy and very obvious visual cues. The overwhelming majority of users would.
Also, if the color schema impedes usability, the problem is the design, not color. Having all of the images in grayscale is great for selling machine parts or technical equipment... And pretty much nothing else. Color is an incredibly important part of how most people choose products. Removing it creates a ton of work for the majority of users who need it. Trying to find one particular type of soap you forgot the name of but it has that green stripe across the label... Forget it. Also, I'm really not sure why so many people have such a hard time acknowledging that branding is an important and legitimate way for companies to communicate about the things they sell, and an important part of how most shoppers orient themselves in marketplaces.
Many people shop without knowing exactly what they need, and displays/carousels/suggestions help people figure it out. That's why they're there.
If visual flourish or whimsy impede usability, that's also a problem with the design and not a problem with visual flourish.
Most people shop at grocery stores instead of discount clubs, and discount clubs instead of restaurant supply warehouses. Why? Most of the products are equivalent or better the further up you go, dramatically cheaper, and presented in a progressively less gussied-up and more information-focused buying experience. Most people in the US have the storage space, too. One big reason is the look and feel. It's just a more pleasant overall experience. Seeing the colorful packages on the shelves, having a more intimate space to shop in with music playing rather than a utilitarian corrugated metal box. Perusing magazines during checkout. That's why Piggly Wiggly easily snatched the market out of the hands of traditional counter service provisioners and small single-product vendors. As a chef, it is the opposite of what I need. Purveyors sold me goods using an interface even more straightforward than McMaster: a printed text list with a place where I could indicate the quantity I needed. Anything more complicated would have been an impediment. Regular shoppers probably wouldn't even consider it.
So this article should be retitled to express why it has the best UX for information-only shoppers who know exactly what they want that are very used to looking at content without dead-obvious visual hierarchy and don't care how anything looks as long as their bullet pointed requirements get met. Like it or not, that doesn't describe the overwhelming majority of shoppers.