rekrunner
Technically, he didn't say they were "important", but "of importance". My English skills are deteriorating, but this sounds "weaker" to me.
Perhaps one way to judge the relative importance of tempo/threshold and long runs, is to observe the number of 10K training plans which don't include them, and evaluate their effectiveness against controls.
For me, this discussion of "importance" is missing a few elements and perspective. What is important is not one type of workout versus another, but developing qualities, like speed, stamina, and endurance. We need to answer questions like what does a 10K race require, what does an athlete need to develop, and which training will bring about the desired changes?
Typical 10K training solutions use tempos and long runs to develop (and maintain) a good foundation, and then finish that training with something like speedwork on the track.