Either way, if he was manipulating her here, why not go with the Palpatine manipulation right off the bat? Pull the same stuff he says in TRoS instead of waiting a year?
Kylo didn’t know by then, Palpatine told him on Exegol. If anything the question becomes why did Palpatine do the waiting.
So then he isn’t lying to manipulate her.
He was using what he knew to manipulate her. Common dark side tactic. An outright lie can be seen through with the force, but something that is true or believed to be true can be a powerful weapon when it is properly twisted.
If it’s not a lie, why bring it up? The whole argument was that Kylo was not being honest, so Rey being a Nobody doesn’t make sense.
The way TLJ is written, the whole “your parents were nobody” was all Kylo trying to turn her. So it being the unvarnished truth doesn’t make a lot of sense.