You, yourself, agreed that any sequence has the same probability of occurrence thus "all heads" is no more improbable than any other sequence including the one that I tossed.
Are you claiming it is impossible to toss any sequence?
The fruitlessness of your attempt to draw the one particular pre-named sequence from even 20 coins would be a marvel to you. And if you had 50 coins -- it would be for all practical intents, impossible.
If it happened -- the more likely assumption would be that you cheated. Even with twenty coins, you're coming back from the lab in under five minutes -- it is certain beyond a reasonable doubt that you cheated.