Well, only two CSU politicians were Kanzlerkandidaten in the past. And indeed, they were either considered too extreme (Franz Josef Strauß in 1980) or too regionalist (Edmund Stoiber in 2002). So the assumption is not entirely unfounded.
But you're absolutely right with your last sentence. Not too long ago, people thought of Söder as a conservative hardliner but this image changed drastically since he was elected Minister-President. He smartly positioned himself against Horst Seehofer (who appeared increasingly clumsy and gaffe-prone), started to praise environmentalism and adopt some ecological positions, softened his stance on immigration, and tried to be perceived as statesmanlike. Of course, this would not have been enough under most circumstances. But then came COVID-19. For Söder, the pandemic was (and still is) an absolute godsend and probably an once-in-a-lifetime chance to reshape his national image.
Also, the risk for the CDU/CSU is pretty limited as they will win the election either way. And Merz (too much baggage), Laschet (too vanilla), Spahn (too young and ambitious) or Röttgen (Who?) have their own issues... so the competition is not exactly fierce. I have no doubt hat the clear favorite would have been Ursula von der Leyen, had she not moved to Brussels.
Didn't he actually get some support as a result of his skeptical/negative stance on Muttis "Wir schaffen das" open-door policy?
Maybe you meant legal immigration, but most people usually don't care that much about legal immigrants coming to fill high-skill or empty positions (truck drivers, welders, etc.)?
