The recent history of executive orders in the US blatantly contradicts this. Sure, you can claim that they have to be supported by law to be put into effect, but that's not actually true: Executive orders are put into place all the time, and if you're lucky the worst ones get suspended by a judge temporarily and are eventually decided in a court hearing months or years later.

The US president can say 'we're doing X' and if the relevant agencies' personnel are willing to listen to him, it'll be in effect the next day. We've seen this with travel bans and family separations.