Would have to see the entire comment in context. It is true that God was a lot harsher toward sinners prior to Jesus’ sacrifice. Indeed, Jesus’ sacrifice illustrates the wrath of God toward sin.
Mosaic law was much more stringent than what supplanted it.
OTOH,
And if anyone will not welcome you or heed your words, shake the dust off your feet when you leave that home or town. Truly I tell you, it will be more bearable for Sodom and Gomorrah on the day of judgment than for that town.
This was a lot different circumstance. In Moses' time the people were purified and made a compact with God to stay true to Him. They were given a mission to preach the Gospel to the world and prepare the way for the Messiah. God does not change but circumstances do. Harshness is from a human perspective besides. God is just and merciful but He has a pathway for all who will listen and watch and those who do not will be cast out.