Not quite: If you have a retail brokerage account you could have put an order in to purchase TSLA anytime this morning when the price was between $18-$20.
When you buy an IPO you have a gentlemens agreement not to sell. (They can't enforce it, but if you violate it you will never get IPO shares again.)
The IPO broker will carefully trickle out shares to the market to make sure of that 40% pop on the first day.
They don't want to release too few or there won't be enough of a market, plus the price will go too high. Too many and the price goes down.
So you don't really get that 40% pop, you have to wait for a more long term price to see how you did.
you said: "The rule is:if you can buy it, then you don't want to."
All I'm pointing out is that you could have bought TSLA at $18 yesterday if you wanted to which appears to violate your 'rule'.