Absolutely yes, if I approached someone with a faith on their death bed, and said "none of it is exists, there is no heaven, you're just going to be gone and you'll never see X loved one again" in a deliberate attempt to upset/distress, and clearly, in that context serving no purpose whatsoever, purely because of a deep dislike I felt for them or one of their characteristics (including for example, if I hated Christians) - then within that very narrow, specific context, that statement would be hateful.
Of course, I'd agree that in general it's not hateful to say "well I don't think you're going to heaven, because heaven doesn't exist" in a large number of different contexts. For example, in a discussion or debate where the existence of an afterlife it's a pertinent, it's not only not hateful to make such a remark, but may actually be a completely necessary thing to say, even potentially achieving aims that could be thought of specifically as kind, such as, wanting someone to make the most of their one and only life on earth instead of making unfair personal sacrifices in the hold out for an afterlife etc.
Just to be clear - I am gender critical (also, incidentally, an atheist) I believe that human beings can't change sex and that actually, the assertion that they can change sex (eg trans ideology) is fundamentally misogynistic (see my first post on this thread!) and not only can, but should, be challenged in most instances.
The only point I'm making is that it is possible to conceive of very specific, narrow circumstances where that isn't the case - which is all I think the previous poster was trying to do. I think actually meaningfully engaging with "are there any circumstances where this is hateful" is quite a useful, clarifying exercise because it reveals, you've got to think quite hard to come up with a scenario where this important statement of fact is actually hateful.