You are correct. The president cannot pardon someone for state crimes. However, Chauvin was also convicted of two federal offenses so Trump could pardon him. But I doubt he will, as that will not really help Chauvin and it will cause an unnecessary fire storm.
Sometimes you have to do what is right, regardless of the outcome. Chauvin was railroaded and all hate crimes and most civil right crimes are crap and need to be abolished.