It's all part of the grift. Like most organizations, the BAA thinks it has to go along with certain types of left-wing identity-centered criticism. If someone says an organization is racist, that organization can ignore the criticism or it can wring its metaphorical hands and say it will "try to do better," but under no circumstances in Current Year can an organization make any counterargument and expect to be able to save its reputation from the fury of very online lefty identity-politics types. See their lame apology statement for this concept in action.
Grifters like Desir know this state of affairs exists in modernity, and that organizations will bend over backward to avoid getting tarred as racist (or sexist, or whateverphobic, or whatever specific grift they are being targeted by today). Most people will also bend over for them, but organizations have more money, reach, and resources. So anyway, the grifters know they can get what they want out of such organizations. What the grifter Desir wanted in this instance, before the race, was to be given an entirely unearned seat at a women's book talk on equal footing with prominent former pros, including ones who had placed highly at this very race. She got it, despite continuing to call the race racist in the leadup to the event . . . see her lame-ass running route graphic that she kept retweeting. After the race (more to the point, after her book talk), what the grifter Desir wanted was to keep the gravy train rolling, so she latched onto the stupid video claiming racism by the Newton Police, in order to keep her name out there and to keep "Boston Marathon = racist" in people's minds. Notice that she didn't retweet the OP video post in that case but made her own new tweet so it could have her name on it (and also the names of her fellow book-talk people, to keep that association fresh in people's minds). Luckily, if the replies to her tweets are any indication, people weren't fooled, even if the the BAA did feel forced to apologize and the former pros did feel compelled to offer support, again mostly for reputation-saving purposes.
Grifter Desir knows there is money and fame available for anyone willing to be the identity-politics activist in relation to a given community, hobby, or activity. She saw that no one was running that grift re: distance running, so she stepped in and took the open spot. She's doing decently well, though I notice she doesn't get a lot of engagement off her own tweets as opposed to ones she piggybacks off of. Probably no one outside of the distance running community knows who she is, so she has work to do. Additionally, distance running is still kind of a niche activity even if it does have broad all-ages participation, so I'm sure she's aiming at using us and our sport as a stepping stone to being able to run the grift in more popular or more heavily-covered activities later on, followed eventually -- if all goes well -- by reaching the top echelon where the grift becomes both general, and highly lucrative: the tier of Coates, Kendi, DiAngelo, etc.