This is a recurrent rumour that pops up over and over again. It is not true.
There have always been people who felt like they were unable to take it, or it was really 120% time or whatever. I've had a variety of 20% projects for years, and I've never worked 120%.
You have to be able to manage your teams and managers expectations, and if you commit to a deadline and fall behind then you're obviously going to have to "bank" that time. If you are in a team or get yourself into a position where you're constantly committing to unrealistic deadlines that have no slack time in them, of course you'll feel like you can't take 20%. But that's not the companies fault.
20% is a great policy. I don't know what else the company could do to make it easier to take really: managers aren't allowed to deny it to you, and you're allowed to bank it up to a reasonable cap if you have to focus on a main project for a while. At that point it's really up to you and your own time management.