Not to mention that people post stuff here ASKING for our opinions.
If I take my product to a public event that I arrange, where I intend to talk to other humans about my product, why is it so unreasonable to assume that I want feedback about my product? What if the person hasn't tried your product but is familiar with other, similar products? Can they use their personal experience to inform their opinion about your product? It sounds like the answer here is 'no.'
It's not like this guy walked into the brewhouse and started giving orders, or even went up to the brewer and started talking shop at him. Since none of us (including the OP) actually know the person we're talking about, why is everyone assuming the worst about this person because of third-hand knowledge?
The crux of this, for me, is that I take umbrage with the idea that every beer is a beautiful snowflake, that requires intimate, personal experience to understand and appreciate, yet a human being can easily be reduced to a caricature without knowing anything about him, except a single thing he said with hardly any context.