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Unlike KA, where you can win with high card alone, unless you hit the flop, both 78s and os are pretty tame hands. And that is the point Gordon was trying to make, many seem to think their chances of hitting a flush increase by 20%: just because your cards are suited, don't assume they are playable/more playable than they really are; and that the chances of hitting a flush are not significantly increased by having to paired hole cards.
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If you hold AK it is correct the suitness does not add much

However with 78 you play to hit the flush or straight, not top pair

Common odds when holding two suited cards:

flopping a flush (including the slight chance of a straight flush in some cases) 0.842%
flopping a four flush 10.944%

Common odds when holding connectors from 54 to JT

flopping a straight (including the slight chance of a straight flush in some cases) 1.306%
flopping an 8 out straight draw* 10.449%

So when playing for str8 or flush only, you almost doubled your chances by playing 78s to 78o