It's a real thing. It's a speed wobble. If you're going fast enough for it to happen, you probably shouldn't be pedaling anyway, so get in your aero tuck and grip the top tube with your knees. This will make the wobble go away. Loosen your grip on the bars too, death grips on the bars can make it worse.

It's normal. In order to build a good race bike, there are certain harmonics that at high speeds will become pronounced. Not much you can do about it, except grip the top tube.