Minato is a top tier lighting-chacrka wielder ninja, it's obvious that he uses his chackra within his kunai.
And anyway tentacles=/= full beast form.
From what I can tell, that's the physical manifestation of the 8 tails's power. And that's assuming that Minato uses chakra on his kunai. Also chakra kunais don't have a good track record of cutting through....well anything really.
Minato's also never shown lightning chakra to my knowledge.
You are assuming that those tentacles have the same durability as the beast, then.
We have no hint, no direct proof that that tentacle is equivalent of the beast's fullpower, while we do have numerous quotes that tell us that the beast is limited from the human in all but the final form.
I am. Cause they look and act completely the same. Also tentacles do tend to have a weakness to cutting things. And actually we have statements that Bijuu are stronger inside hosts.
Wait wait wait.
So you're just going to avoid the massive size difference and the slight color one the tentacles have while changing form.