It is possible that Christ was distinguishing between authority and practice. In the previous chapter He had just confronted the Pharisees and exposed much of their hypocrisy and lack of faith. Their purely intellectual approach to matters of spiritual concern made them singularly inappropriate as mentors to a people seeking value based on such spirituality.
The crowds had heard this exchange, and now in Chp 23, Jesus turns to them and warns them about the things that needed to be avoided. The authority of the Pharisees was not a matter of concern to Him, since He was neither Anarchist or impractical Rebel. The Seat of Moses was both a reality and a metaphor. A recent archaeological find at a site called Chorazin [mentioned in the NT] has unearthed almost an entire 1C synagogue. You can clearly see benches crafted into the side of the structure placed their for dignitaries, while the general assembly may have sat on mats or carpets. From that position of authority the attenuated speaker would expound on "Moses" or the Torah, hence the development of the metaphor "speaking from Moses' seat"
Many of these Pharisees saw themselves as the Good Guys. They perceived themselves as holding the line against radicals, and liberals, like the despised Sadducees, and all manner of anti-Torah misfits. This was not the matter of concern to Christ.
What they taught, and what they practiced, accouterments that developed from their teaching, was indeed a concern. When they spoke on the Law they were fulfilling a function. Thus Jesus said: "All they tell you [which is from the Torah, expounded from the Seat of Moses, is good, so] observe it". However their own teachings, which were an unnecessary addition to the Torah, and which constituted burdens to their hearers, were products of themselves and did not constitute righteousness. These practices were what Christ condemned.
Cheers