This is not true. I can call myself a doctor a lawyer or a cop or anything like that and it is protected speech so long as I am not attempting to perform the professional duties of that job
It actually is true, unless MN has weird rules compared to other states. I'm not a lawyer, but the code here, sec. 326.02 seems pretty clear.
or to use in connection with the person's name, or to otherwise assume, use or advertise any title or description tending to convey the impression that the person is an architect, professional engineer (hereinafter called engineer), land surveyor, landscape architect, professional geoscientist (hereinafter called geoscientist), or certified interior designer, unless such person is qualified by licensure or certification under sections 326.02 to 326.15.
You actually can't call yourself a professional engineer if you're not - theres several lrgal cases where i am that are ongoing due to people calling themselves engineers while being realtors, for example, and trying to use the title to advertise (IE John Doe, P.Eng), which is not allowed.