I don't know if I agree with "dangerous", but I agree in spirit. Basically, the DoJ said "Trump asked for it in public, but he didn't mean it, so we're not doing it". Basically, I think it comes down to "Trump is lying to the American people, but not to us specifically".
The DoJ admitting Trump is lying is just getting on the same plane of reality as everyone else. I know what you meant by "dangerous" but if anything, this seems Trump-specific and unlikely to be used in the future. After all, name one time W or Obama gave orders through social media.