It is according to US law. Without a larger percentage, no court will consider anti-trust charges, due to Circuit and Supreme Court cases on what percentage of market control is required. Here's relevant portions from DoJ:
"In determining whether a competitor possesses monopoly power in a relevant market, courts typically begin by looking at the firm's market share.(18) "
So the typical case starts with percent control....
"Although the courts "have not yet identified a precise level at which monopoly power will be inferred,"(19) they have demanded a dominant market share"
So the courts require a threshold.....
"the Tenth Circuit noted that to establish "monopoly power, lower courts generally require a minimum market share of between 70% and 80%.""
There's ample more court citations on the DoJ page.
Amazon doesn't reach these levels in probably any relevant category they are in. Certainly not for retail sales (Amazon is a tiny portion there), or online sales (less than half), or web services (Amazon ~ 40B out of 350B market).
For any company, if you shrink the categories far enough, you can always find a monopoly - Apply has a (near) monopoly on Apple watches, for example. But this is not a monopoly.
https://www.justice.gov/atr/competition-and-monopoly-single-...
being a platform business that competes directly with its business customers can be a significant (but not necessarily sufficient) indicator as well.