That's a different issue. Presidental succession says who replaces Biden if he dies in office, but the question here is who would stand as the Democratic candidate for the 2024 election if he died.

Harris would be likely, but maybe not a 100% certainty.

Elections have nothing to do with presidential succession. It will be a party decision, Harris is a guaranteed loss, and M. Obama is a possible win.

Lines of succession are a separate thing from winners of elections. If Biden were to die before election day, that doesn't mean Harris would be on the ticket as the presidential nominee. I'd wager against that for sure.

So who would be tapped to fill in for the recently-deceased candidate? Could be Harris, or Obama, or a dozen other hopefuls.

In the minds of conservatives and some leftists, the concept that Kamala being unpopular is so ingrained that they cannot believe that she would be the nominee no matter what the circumstances. In reality she's not much more unpopular than her President and would likely see a massive approval bump if the worst happens to the current POTUS.

Even without that, she's the odds-on favorite to be the nominee in 2028 by incumbency alone. Basically anyone that follows politics understands this, but the silly conspiracies are based on motivated reasoning.