"There does seem to be some thinking that "black lives matter", means "only" rather than "as well"."
As an English speaker for many many years, I can assure folk "only" is exactly what is implied. Otherwise the colour part wouldn't be mentioned. It's an exclusive statement. This group matters, not the rest who we don't care about. Folk who thought it up would have been adult enough to know full well what they were stating and purposely chose to use it.
That pointed out, this is about criminal damage not the feeble excuse used in vain to try to justify it.