That's exactly the point he makes, you're misunderstanding him. Holding mana for Spell Pierce open would be wasted if the opponent plays a creature (because it cannot be countered by it). In that case you spend your mana by casting Opt and do not lose out on tempo.
He just pointed out the instant speed synergy with counterspells - something that Serum Visions does not provide. Same goes for Spell Snare, obviously (if the opponent does not play a 2cmc card).