I didn't down-vote, but I expect the comment comes across as antagonistic. Objectively, he isn't whispering. Whispering means sounds are not voiced, which is not the case here. So at best this is an exaggeration. Bluntly stated as it is, it likely seems unnecessarily negative in tone to some.

Tone is interpretation. I think we should prefer to error on the side of doubt before down-voting.

Let's assume they didn't down-vote because of interpreted tone but instead because they disagreed on the statement that the speaker whispered.

Why don't they put forth an perfectly good, objective criticism like you just did with

    Objectively, he isn't whispering. Whispering means sounds
    are not voiced, which is not the case here.

instead of down-voting?

I think down-voting should not be used for expressing disagreement.

Destructive behaviour is a good reason for down-voting.

Was his comment destructive? He used the word "whisper" non-conformant with the definition you brought. I don't think that was intentional and I think it was easy to get what he was trying to say. Other people also noted the "golf commentator voice". His behaviour was not destructive.

Down-voting is misused. I don't know how the community feels about this, but I see it as an issue.