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Why would that be a sign of a bubble and not simply an overvalued stock?

if apple were worth more than all of the other personal computer makers combined (maybe it is idk) would that also be a sign of a bubble? is that all it takes or does growth rate matter?

Agreed, but remember that if you are thinking about betting against Tesla that the market can stay (much) longer irrational than you can stay solvent. There is a quote from the early 20th century: "Wall Street's graveyards are filled with men who were right too soon."

When VW stocks went through the roof due to a short squeeze a few years ago, a very conservative German billionaire named Adolf Merckle saw that VW's stock value was ridicilously overpriced, so he decided to bet agains the stock. But the stock price went further up and stayed there long enough to make him lose all of his money. He then committed suicide.