The last 3 successful Democratic candidates before Biden (Carter, B. Clinton, Obama) were all national unknowns with personal charisma who ran as (for their time) center-left candidates.
Phillips' flipping of a congressional seat in a district that had elected Republicans since 1960, then winning reelection by increasing margins for the next two elections, proves that he can effectively win over and retain swing voters in a center-right electorate.
Writing off Phillips while assuming Newsom would be a great general election candidate seems a bit weird when held up against history.
So, first of all, no, they weren't. Jimmy Carter was the closest thing to unknown, but he was still governor of a state. He won as a dark horse in a year that a stick painted blue would have won the presidency. Bill Clinton was governor of Arkansas, and had been for 8 years. He may have been unknown to the general public, but he wasn't unknown to the average Democrat, having been touted as a possible candidate in 1988 and having given a keynote address at the 1988 DNC. He spent 4 years building his national profile so that when he entered the 1992 primaries, he was well known by Democratic party voters, who very nearly rejected him anyway. Obama had very aggressively raised his national profile as well; he was given the keynote address at the 2004 Democratic National Convention and then as Senator basically spent the whole time running for President, even if he wasn't yet running for president, if you know what I mean.
Obama didn't run as a centre-left politician in most ways. He campaigned on public health care, which is absolutely not a centre-left ideal. Clinton and Carter were more centre-left, of course, but that was in a much different time. The centre is very nearly dead. The GOP has realized this. The Dems need to.
Dean Phillips, by comparison, is an unknown to most Democrats, even wonky folks. Not as unknown as Mike Johnson (lol) was three weeks ago, mind you. He has no national profile, very few national alliances with other politicians. He has a nice feather in his cap - winning a district that was held by Republicans for a very long time. He has sponsored five bills that passed the House since he has been Congress. Two became law. One was to rename a post office. One was to extend the time businesses had to repay PPP loans. Hardly something for the average Democrat to run on.
I'm absolutely not writing off Phillips as a campaigner. I don't know how skilled he is at national campaigning because he's never done it. He's campaigned in the Minneapolis suburbs, and by all accounts, extremely well. That's significantly different to campaigning across the USA. Newsom has been quietly campaigning for years and he's good at it. He has supporters in the Dems across the country, not just in California, and he's good at getting on TV. Meanwhile, Phillips is trying to stab the sitting president in the back. Even with Biden's weaknesses, that's not going to earn him any friends in the establishment, nor is it going to earn him any friends among other Democratic lawmakers. Even the ones who don't like Biden. Look at what happened to Eugene McCarthy. He did the same thing - tried to beat LBJ in NH, and almost pulled it off and humiliated LBJ. He ended up not running for re-election to the Senate and endorsed Reagan.