I'm not 100% sold on that being true. I think people accept or reject new ideas based on how well such ideas conform to their values or preconceptions or self-interest.
Most of that stuff is maturation. Elu Thingol was fully mature and hanging around with the parents of his future Noldor neighbors. I don't see how having one kid changes his language. I could see if he came out of the woods with Melian and was suddenly speaking a new tongue, given other changes at the time, but the evolution of the language he and his subjects speak doesn't make sense as an evolution of his personality. Especially given the sheer lack of anything going on at the time that might inflict such changes.

But most of those events happen after Sindarin had evolved. Men woke when the Sun showed up, and that happened around the time the Noldor arrived in Middle Earth. The Noldor came in hot pursuit of Morgoth, so even though he had time to initiate the Wars of Beleriand before they got there, it could not have been long enough for the Sinder to change. The only major noteworthy event following Thingol's marriage & decision to stay in Middle Earth was the arrival of the Dwarves in Beleriand under circumstances that deny any linguistic influence. I could see if Sindarin had changed so that what was spoken during the War of the Ring was radically different from whatever they spoke in Beleriand, but not under the relatively uneventful circumstances prior to Morgoth & the Noldors' return.

We might not know much about immortals, but in Tolkien, they are pretty much in stasis over the years, only changing in response to certain events or knowledge.

If there was enough change of a person over several millennia that his primary language evolved, the fact that the sons and daughters of Finarfn are also children of a niece he never met should not have mattered to him. Hell, a reunion with Olwe himself should have been a matter of indifference, because they would have been radically different people. But Thingol welcomes Finrod and Galadriel and their brothers like his separation from their grandfather was a relatively recent and aberrant thing in their lives and even when his pissed at their proximity to the Kinslaying and their concealment of it, he still is open to maintaining the family ties. Those should have been part of another person.