"You aren't doing it right" is a valid argument when "it" is well defined. No True Scotsman is a fallacy of ambiguity—it occurs when "Scotsman" isn't well defined, allowing somebody to continually move the goalposts in response to an argument.

In the case of Agile, Scrum, and retrospectives, they all have published definitions, so No True Scotsman doesn't apply.

Are you valuing processes and tools over individuals and interactions? You aren't doing Agile right. (Agile Manifesto)

Does your team not consist of "one Scrum Master, one Product Owner, and Developers [with] no sub-teams or hierarchies [who] internally decide who does what, when, and how?" You aren't doing Scrum right. (Scrum Guide)

Does your retrospective not include "Decide what to do?" You aren't doing retrospectives right. (Agile Retrospectives book.)

In fairness hardly anybody does these things right, so it's tempting to lob No True Scotsman criticisms. But that's just as fallacious. A more accurate criticism might be to say that Agile & co. are too hard to do right, or to say that they require a business environment that doesn't exist in most companies.