it's also good not to forget that kushina could have, for any intentional reason, gone into any stage of tailed beast mode and allowed kyubi to use its power (beast bomb). it's likely minato would be close. what is almost certain is that any beast bomb minato would have witnessed from the kyubi would not be in the context or scenario of an attack on konaha or else it would have been something mentioned already. maybe minato and kushina fought side by side and used the kyubi's power a bit. they could have even been battling another beast or jinchuuriki.