An establishing shot to make Kylo look intimidating by hindering a single soldier's attack isn't really suggestive at what Luke can and can't do when facing off against a massive army,The Sequel Trilogy opens with Kylo Ren Force stopping a laser bolt in mid-air.
But at any rate, it doesn't really matter:
It sells that Luke's mythical reputation is strong to those within the Star Wars universe.
Kylo doesn't believe it because he has a good reason to think that Luke could do it. He believes it because he doesn't know what to think.