Pointing out that those venues were full of anti-everything hate is not dismissing a black person's experience, it's validating it, while pointing out that he's not alone.
An abuser on IRC calling a black person the n-word is objectively more harmful than an abuser on IRC calling pretty much anyone else pretty much any slur.
For instance, a black person might be reasonably angry at hearing the n-word, until they find out the person suffers from Tourette's, at which point the "harm" vanishes. The harm clearly did not result from the word but from the framing of the situation, and the framing exists entirely within the mind and can be changed.
This pattern of harm resulting from framing is seen repeatedly throughout psychology. Of course we should still push for relegating incorrect and outdated framings, like the various -isms, to the dustbin of history, but there's considerably more latitude for how to do this than naive language policing like is commonly suggested.
But it implicitly implies the negative experiences were similar in scale unless otherwise noted.
It downplays the very large weight that racism adds to the negativity of that experience even if the intent is a noble one of making that person not feel alone.
Do you not see how that response is problematic? How it downplays the person's experience?