Hmm... this sounds an awful lot like, "because you're responding to a comment made by a black person, the only acceptable response is one of total agreement". I think you might want to reflect for a minute on the importance of nuance in a discussion, and the net effect of trying to police comments on others' behalf.
I completely believe the commenter's experience, but I also witnessed the opposite a lot (and was deeply encouraged by it), and it's not wrong for me to chime in and point it out. And guess what? Experiencing vitriol on the internet directly contributed to me being more sensitive to things like racism. Although I've been on the receiving end of racist remarks offline, it wasn't until the internet that I caught a glimpse at how hurtful people could truly be. Again, not wrong for me to pipe up about it.
I completely believe the commenter's experience, but I also witnessed the opposite a lot (and was deeply encouraged by it), and it's not wrong for me to chime in and point it out. And guess what? Experiencing vitriol on the internet directly contributed to me being more sensitive to things like racism. Although I've been on the receiving end of racist remarks offline, it wasn't until the internet that I caught a glimpse at how hurtful people could truly be. Again, not wrong for me to pipe up about it.