Yeah that was confusing. I see where the logic comes from, it's just explained wrong. I think it's what Ace explained as the root is usually on the bass guitar. That's why it's not in the chord. But any chord can be substituted with another chord, which is another thing that person was trying to explain.

However I don't agree with the idea that the root is the least important note in a chord. They are all evenly important to this concept.

In fact the most important notes are the root and the third when it comes to basic substitution because the relationship between the root and third is what makes up major and minor chords. Like the notes of C major, C and E. Any chord with C and E in it can sub for the C major chord. Typical example is A minor.

Another common subsititution is for the dominant 7 chord, which is (not the tri-tone bII7) but VIIo7 chord. Which is handy because this chord is a powerful modulation tool. Since any note in this chord can act as the root, or leading tone to another key. Usually this sub is not seen this way, but instead as a V7b9 chord, and almost always in first inversion (3rd in the bass).