Was reading through his wiki page and saw it written there. I thought it was a load of nonsense, but then I watched the episode again and even read the manga panels of when he did it, and the wiki is correct.
You must be still to gather nature energy. Minato threw his kunai at Madara's feet and then entered sage mode in the one moment. No wonder it didn't last long, here is no way he could have gathered enough nature energy in just mere seconds to have actually maintained the mode.
I find is quite impressive to be honest, Hashirama is the one who is known to be able to enter sage mode instantly. However the way Minato utilises it would be extremely dangerous in battle. I could see him using it to land a fast killer blow on his opponents.