As he likes to remind people, Jim is a professional liar, so no.
That being said Chandler is a pretty minor character, so I don't find it that farfetched that Jim doesn't have a concrete idea of what happened to him, rather then just vague ideas.
Jim also said in that interview part of the point of Battle Ground was clearing some characters out. Too many for him to keep track of.
It's just that the description of what happened (the black square) was so precise, I cannot imagine JB thinking that and not thinking what it means. Perhaps he is just trying to downplay what could be something important.
Instead, for some reason, I believe that he is not sure about Thomas. There are many options and I can see him not being sure what to choose yet. Personally, I hope he is back soon because I love Harry & Thomas together. And I need to see him taking care of his child soon because of the family drama annoys me. That also means I want Harry and Lara rescuing the child soon.
Black circle actually, but I get your point. I mean, it could lead to anywhere. Some horrible part of the Nevernever. The Outside. Drakul's lair. Maybe Jim, like many authors, likes leaving his options open. More interesting for him.
Thomas is sadly being sidelined as Jim is bored/tired of him. Which is such a waste to me. He tried to say Harry just thought it was cool having a brother out of nowhere, rather than the massive sense of family he got by getting a brother. First Harry flipped out, then he felt relief he wasn't alone, and then a deep love. I get that Thomas serves a narrative purpose but Jim can change the character and grow him. He's the author after all. I think Jim just can't be bothered as he no longer finds the character interesting or useful, and he takes up space. Doesn't mean he won't come back of course. Tbh I am surprised Jim didn't just kill him (which he may well do).