owning digital currencies is not even remotely "one of the worst things a startup can do with their money". it is not an investment. it is not productive. but it is not a bad idea.
In other words, it's a gamble. And it has terrible effects on the public image of the company if you ask me. If I was a stockholder, that would make me question the long-term stability of the company, if the executive has tendencies towards such behavior.
I'm only half joking when I say this but I think you're neglecting to consider that the Venn diagram of investors/speculators bullish on Tesla and bullish on BTC is a single circle.