It's a relevant statistic for this discussion, which is not a discussion about generic police brutality. Retracting from "Black Live Matters" to "we're discussing police brutality here" is a common mote-and-bailey tactic that isn't a valid argument.
Moat-and-bailey is perfectly valid if it's used to indicate how failure of the argument to address the more-defensible position implies the argument fails on the less-defensible position (as a general comment on rhetorical process, not a comment on Black Lives Matter or police brutality).
