I don't understand why it's necessary to bring this up or its relation at all with SSC or the New Yorker piece.

I am speculating on behalf of GP, but I think the reason it might be relevant is because the New Yorker has a far-left bias (https://www.allsides.com/media-bias/media-bias-chart) and the motivation to go after SSC in the first place (for some on the left) seems to be that SSC's author and comments on SSC were willing to hold objective conversations about race and other touchy matters that the far left often dismisses as off limits (for instance using the label of "scientific racism"). This Harper's letter is in support of open debate, free inquiry, and free thought, and therefore it is in effect in support of SSC.

Free thought and free inquiry and the freedom to research/discuss/publish unpopular ideas has been a hot topic in recent times. For instance, see the protest of Omar Wasow's paper on protesting (https://www.chronicle.com/article/The-Protesting-of-a-Protes...) or the firing of Steven Hsu (https://quillette.com/2020/07/01/on-steve-hsu-and-the-campai...).