He does look at Wei Ying when he confesses this, staring him up and down as if trying to discern if he is lying in order to - in order to what, make Lan Wangji feel better? No, he wouldn't do that. He's certainly telling the truth.

But - but Wei Ying always flirted with others, didn't he? Always talked a big game.

He turns back towards the screen as the protagonist picks out a dress with the rival love interest and discovers she might be falling in love.

Lan Wangji takes the tub of popcorn, and places it onto Wei Ying's lap. He takes the ridiculously large drink they've been given, and places it on the armrest between them. And in doing so - because these armrests are quite small even though these chairs are quite spacious, he does brush against Wei Ying's hands again. He doesn't jerk away this time, but he doesn't react at all.