What’s the basis for that assertion?

Dunno.

But I think that tweeter might be reading into Amy's exchanges with Mayor Pete in the last debate.
She certainly confronted him over what she feels is his lack of experience. She also called him out for what she felt was his down playing the experience of other candidates on the stage that have won state-wide office.

But who knows...

Many of them may very well see Buttigieg as their second choice.

They do value her opinion after all. She made a point about Mayor Pete's lack of experience, so if they agree with that at all then Biden seems the likely candidate for them to shift to since he has tons of experience.

Time will tell.

Personally, I wish this wasn't true because the point that Biden will benefit really hurts the chances of my preferred candidate. I was hoping that Mayor Pete would siphon support away from Biden leaving Joe to finish in fourth place.

But that seems unlikely now.

I didn't consider the implications of this before.

They are both Midwesterners operating in the Moderate Lane ….If she stays at 7% , that vote will be evenly divided between Biden, Pete and Warren … ...She will reach 15% in the areas close to the Minnesota border ...The three candidates mentioned above will be dividing up around 4% - 5% , not the entire 7% …..Bernie and Warren have the enthusiasm edge , according to the latest CBS News Battleground Tracker Poll : Voters enthusiastic about vote choice : Bernie 67%.. Warren 61% ….Pete 52% … Joe 49% …..Organization and Enthusiasm carry the day in a Caucus ...

Who Amy will be targeting, but in the Minnesota nicest way possible.
Five on the stage is even more decisive then the last one and no one can hide so it will be very dynamic.

I doubt their spat at the debate changed anything, other than give Amy a boost. They are now my top 2 by quite a margin. If you want a sensible candidate who's not way too old, they're really the only ones left.