The headline in the tweet with "a #BLM advocate," refers to this one BLM advocate. https://nationalfile.com/revealed-a...ite-childs-death-after-jan-6-capitol-protest/ Indefinite article lives matter. http://partofspeech.org/what-part-of-speech-is-a/ And the thread title, "BLM Advocate beats child to death," also refers to the same BLM advocate, not all of them.
Thank God I live rent free in your head! You never did tell us what it is you do. You must have missed that question.