Anyone else cringed at 4:00 section because of parallel octaves between lead guitar and acoustic? How could educated musicians make such a big harmony related mistake?
why do you "cringe" at 4:00? What's so wrong? What do you think they should have done differently?
I'll try to explain it easily. When you have two voices playing perfect consonances (fourth, fifth, octave or any of these displaced by octave(s)) in parallel motion, the supposed-to-be different voices sound dependent to each other. To a (educated) musician, it's very obvious when you hear it, but it would probably also sound better to you if there was a ''correctly written'' version for you to compare. That is an example of a bad voice leading (voice leading - progression of individual melodic lines and their interaction to create harmonies) because the voices are not progressing individually; it basically ''kills'' the purpose of the second voice (acoustic guitar in this case). Reason for this dependence lies in acoustics - octave is the 1st overtone of fundamental note, fifth is 2nd (and fourth is just inversion of fifth), so they are very closely related to the fundamental.
Sometimes, these consecutive perfect intervals are created on purpose (like power chords, or unisons between keyboard and guitar in many DT solos) but in these cases, these either aren't really two different voices (power chords), or it's written like that on purpose to create a specific tension that needs to be resolved (like jumping from unison to tritone interval between keyboard, guitar and bass in Octavarium (at 17:00)). In this case, however, we're having just a normal song so consecutive octaves are completely unjustified (since some people said they did it on purpose). It would be just like a song like spirit carries on stops before chorus, JLB farts into the microphone, and the song continues. They wouldn't do it on purpose because it's not in the spirit of the song. This time, however, I'm sure it slipped unintentionally.
Do you mean that they should've made the harmony more "difficult" just because they have the knowledge on how to do it?
It's not the point of making it more difficult, they could've actually made the acoustic guitar play just chords instead of the melody and it would've fitted better. You would fail a harmony exam if you wrote a voice leading like this. For the rest of the answer, refer to my answer to the previous question in this comment