I would say that "low-key" is a strange phrase to use, as it's essentially jazz slang (like a low, bass note), while also somewhat outdated. "Low-key" would be how you would ask for drugs at a party or a show. You want to buy drugs, but you don't necessarily want everyone to know you want to buy drugs, so you ask the right person who knows the guy that knows the guy - perhaps you whispered your request all, "low-key" like.

I would sort of agree that it's a strange place to put it in this sentence. It kinda needs an em-dash or something:

"Walton knew: the bicycle is -- low-key -- one of humankind’s greatest inventions"

But I would edit that all out as it's not needed and disturbs the flow of the sentence.

I think what the author is trying to say is more, "underappreciated".

Walton knew: the bicycle is one of humankind’s most underappreciated, yet greatest inventions.

Perhaps in the vein of indoor plumbing, tho many people go without riding bikes, while none of us who have indoor plumbing would give it up.

Or maybe even "under the radar". You know the best place to eat in town, but it's just some hole in the wall managed by a single guy, but god-damn do those mashed potatoes slap.

Bicycle love isn't something advertised, like a car is. You get on a bike and you fall in love with it. Attraction over promotion.