So what? Rayleigh doesn't have a df and yet he is still on par with a Kizaru who has one of the strongest df around. Being held down by Sengoku doesn't mean shit really. Garp wanted to kill Akainu by sudden burst of emotion but at the same time he knew in his mind that Ace will end up dead anyway and what Akainu did was just following orders to execute Ace so he had this internal struggle to control himself or go apeshit on Akainu. If he really wanted to kill him he would have fought Sengoku to free himself which he didn't.
Remember during Rogers era, WB and Garp are the only ones who can fight Roger on the same level. It's safe to assume that the power diffirence between the two ain't that big. Now, old WB who has a stabbed on his chest and has a great illness still manages to beat an Admiral, just imagine if that were Garp who doesn't have any illness we know of and probably younger than old WB by a couple of years (Garp still has dark hair at the time of Roger while WB looks just as old as now). I honestly think that Garp would have murdered Akainu with no problem if Sengoku didn't interfere. The same Admiral who the whole BB pirates ran away from.