When I used the Wyeast lacto culture I don't recall seeing any visible signs of fermentation. As such, my concern would be that you got some wild yeasties in there. If that's the case, then at least pitching might give your yeast a chance to beat out the wild yeasts before they do too much damage. On the other hand, maybe what you thought was yeast activity really was the bacteria and for some reason it just looked different than what I recall from my experience. Either way I think I would've pitched the yeast too if I were you.