Not speaking for GP or the way they meant to use the language, but a monoculture doesn't have to be a monopoly. It just has to be the only crop you grow on a given field, or in this case, the only EDR tool in use at a given company.

The fact that this took out whole clusters at some companies, or nearly all DCs at some companies, is part of what made it so hard to recover from for companies hit by it. That is the metaphorical 'monoculture', not CrowdStrike's market position.