In theory yes. However, I've taken cranks out of perfectly good running motors only to find that when I do my WD test it very slowly seeps out.

It could be that when up to running temp the seal softens up and the spring tension helps seal it.

I've also performed the test that you mention, about filling one case with fuel/oil and seeing if it leaks, and I've seen a couple that will weep a little of the mixture over time, but again, once running they work fine.

Just goes to show that up to a point, the good old NSR is self healing......maybe!!

I've also seen bad running engines with a centre bearing that spins without any drag and leaks like a sieve. Throw in a new crank and you're ready to rock.