Q: Is their defensive ability better enough than the next three more two-way blueliners to justify such early selection?
A: Maybe, maybe not.
This is the only thing that caught my eye this weekend, aside from the mistake of drafting Luc Robitaille a) so early, b) for chemistry he supposedly, but didn't actually, have with Gretzky, and c) to create the softest first line in ATD history.
I think there might be some misconceptions about the offensive values of these players. First of all, in a real life sense, they are
all two-way defensemen. It's only when they "graduate" up to the ATD when the question must be asked about whether they would begin to specialize more at one end of the ice or the other. It's reasonable to begin by assuming the defensemen with the best offensive resumes will continue to be relied on more by their ATD teams for offense.
Here are the six defensemen you mentioned, ranked by their VsX score for defensemen. Just this morning I did the pre-merger leagues so that Cleghorn could be analyzed in the same way. With the multiple leagues going on at once for 16 years, it's often difficult to determine just who should be the benchmark player and/or what the benchmark number in each league should be, further confounded by the fact that we're not always 100% sure who played defense in which seasons (though we're 90% sure Cleghorn himself never played anything but D/P/CP). As far as Cleghorn is concerned, there's definitely some "margin of error" there because of this, but I also think he's so far ahead of this group offensively so as to negate this factor.
These are the six defensemen you mentioned, ranked by 7-year VsX for defensemen (10 year score in parentheses):
Sprague Cleghorn: 743
Chris Pronger: 622
Earl Seibert: 620
Tim Horton: 608
Scott Stevens: 576
Zdeno Chara: 540
So I would not categorize Chara, Pronger and Stevens as the three best offensive weapons in the group at all. Pronger is very much "in the pack" and Stevens/Chara quite clearly were the least dominant offensive players of the bunch within their eras.
It's really easy, even for experienced people like us, to get sucked in by the higher numbers posted by defensemen post-Orr, but there were plenty of defensemen before him who distanced themselves from the pack consistently and should be recognized for the stronger puck movers they were.