Sub-6 has a low frequency, so it travels farther, so the cost of building a 5G base station is low, and the distance between stations can be farther.
Millimeter wave technology and Sub-6 are both 5G standards specified by 3GPP, and both are true 5G. Sub-6 can also be called centimeter wave, because the wavelength is centimeter, and millimeter wave means millimeter wave.