Hmm I don't know. Are they coming to those conclusions solely by Marsden replying with "Circle of life my friend. Circle of life"?

While I would believe that Kovu might not appear in the show (for whatever dumb reason Disney might give), he might as well be just saying he won't be reprising his role as Kovu, giving the part to someone else.
Or even if he meant that Kovu won't be in the show, I doubt it'll be because he died. I think that's a very early conclusion based on just that little reply.

Although I admit, it would be interesting if it played out like that. Heart crushing, but interesting nonetheless. Maybe the TV movie could show us Kovu's demise... but yeah, that would be a bit too heavy for Disney Junior xD