He hasn't been able to hear the entire conversation at this distance (except for Wei Ying, who is quite loud), and Wen Ning speaks quietly, but Wangji continues to have suspicions about Wen Ning being from the future as well. Nothing he sees or hears makes him suspect this less.
It's bittersweet to see Wen Ning and Wei Ying together, both young and happy and allowed to exist without being hunted. With Jiang Wanyin, who doesn't want to kill them, and Jiang Yanli, who isn't going to die. (She can't die. It is very very important that Jiang Yanli does not die.)
Wangji is slightly jealous at how easily Wen Ning gets to go right back to being Wei Ying's friend, laughing with him and earning his warmth and smiling at each other. Wangji doesn't know how to do that. How to just... be friends. Wei Ying needs to follow him around for months to get that same comfort and closeness with each other that Wangji misses so badly. He doesn't know any other way to be known by him.
If Wangji isn't around or if Wei Ying doesn't feel so inclined, this time, then - then that'll be that.