I'm sorry, what? The line of presidential succession has been long established, it's in the constitution, and Kamala Harris is next in line. And then a lot of people who aren't Obama.
So who would be tapped to fill in for the recently-deceased candidate? Could be Harris, or Obama, or a dozen other hopefuls.
Harris would be likely, but maybe not a 100% certainty.
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.