"Police power" is not the authority to send people out in blue uniforms with badges and guns to enforce the law. The Supreme Court defines it like this:
The police power of a State embraces such reasonable regulations relating to matters completely within its territory, and not affecting the people of other States, established directly by legislative enactment, as will protect the public health and safety.
Jacobson v. Commonwealth of Massachusetts, 197 U.S. 11 (1905).
This often includes the authority to send out law enforcement with guns, but it also includes all manner of more mundane laws, such as establishing speed limits, building codes, and sewer systems.
In this sense, it is essentially fair to say that the Tenth Amendment reserves the police power to the states, as the federal government is not allowed to just make any law it thinks would be good for the public, though it may do so when they law is within the scope of one of us enumerated powers.
So the federal government may not exercise a general police power" in the technical sense, but there is not really any vertical separation-of-powers issue when it creates a police force like the FBI or ATF, as long as those officers are staying within their jurisdiction and enforcing laws within their grant of authority, which could be to carry out any of the enumerated powers. This could mean addressing issues of interstate commerce, but it could also mean making sure no one besides the government is using currency (Secret Service), or that people are paying their taxes (IRS), or whatever else.
And because the enumerated powers actually belong to Congress, there is no horizontal separation-of-powers issue when Congress creates a police force to protect itself and enforce its own orders.