To be fair, I never played amplitude and after watching the video still don't understand how it's played. Messaging is really off on this one. They should have showed how awesome the gameplay is. I still backed, only because I want more games on my ps4 and because I loved rock band.
It's not all that hard to understand when you play it, but it goes a bit like this.
You have three buttons and three "notes" on the lines. The left note is played with either L1 or Square, the middle one with R1 or Triangle and the rightmost one with R2 or Circle. Now, the idea of the game is that there are several lines filled with notes that you have to play and you can jump between the lines completely freely. One line might be for drums, one for vocals, two for guitars and one for piano (or whatever the song needs). Once you've picked a line, you have to succeed playing all the notes of two bars in succession and once you do that, you get a bunch of points, a boost to your score multiplier and that line will be "activated" (say you succeed in hitting all the notes on the drum line, the drum beat will start to automatically play in the background for a while) and that line disappears for a while and you have to move on to the next line/instrument.
The idea is to activate all of these lines one at a time and if you succeed in that without any screw ups, more & more parts of the songs start to play in the background (which is really satisfying to hear), your score multiplier improves and at some point you'll usually have all the lines activated for a little while and you hear the full song playing in the background. Then the lines start to come back one by one. So the idea of the game is to try to keep the lines activated & playing as efficiently as possible.
There are a couple of things that bring some depth to Frequency & Amplitude. One of those are the power-ups. You have score-doublers, auto-activations and the freestyle dj-thing that can give you mad points if your score multiplier is maxed and you have the score-doubler activated at the same time. The idea is to try to find the best moments to activate them. A score-doubler might be wasted at some parts of the song. The second one is that while it might seem logical to go through the activation of the lines from left to right and then jump back to the leftmost line once it comes back again (usually it's the first one to come back after succeeding all the lines in a row), that's not necessarily the best "route" through the song as far as high scores go. Sometimes you might want to skip a few bars, even at the expense of your score multiplier, if that helps you get to some particularly score-rich lines later on (whereas the logical left-to-right way might give you simple lines that don't give you much points no matter how high the multiplier) or helps you get some power-ups that help you get mad points. Sometimes you might want to skip a line and delay playing it for better scores even if it's the next line going from left to right.
And while the three button gameplay sounds quite simple, there's a bit more to it too, especially in harder songs. I mean it in a way that some songs might have a quick succession of left left left left notes that are somewhat hard, impossible or at least really cumbersome to play if you only play with the shoulder buttons or only with the face buttons, so for the harder songs you will have to play a "LeftLeftLeftLeft" group of notes by pressing L1-Square-L1-Square. Doesn't sound too difficult, but it takes some adjustment to master.