The difference in US law falls under the Posse Comitatus Act[1] (which has been repeatedly expanded over the last 150 years): the government, by law, cannot use military forces for domestic law enforcement.
I'm no particular fan of the US military (much less the police), but the practical and historical distinctions between the two are substantial: one is the professional military of a country, and the other is a professionalization of antebellum slave catching posses.
I think the comment you are replying to is making a slightly different point: what's the point of Posse Comitatus when heavily armed, trigger happy, indemnified government employees can do the exact same thing by just wearing blue instead of camo?
That limits what the federal government can do with the US military. Governors have more lattiude with using state-level military (i.e. national guard units) in their own states.
I'm no particular fan of the US military (much less the police), but the practical and historical distinctions between the two are substantial: one is the professional military of a country, and the other is a professionalization of antebellum slave catching posses.
[1]: https://en.wikipedia.org/wiki/Posse_Comitatus_Act