I found miguelgrado's comment fascinating:
if you go to the IExpolore.exe file itself, and right-click on Properties > Details, one sees that the file & product version listed there is indeed 10.0.9200.16611
yet despite this, when one runs IE and clicks on Help > About Internet Explorer, we are told that the version we're running is 10.0.9200.16618
In other words, there's an inconsistency in the information reported by Microsoft. And based on the first result, that may explain why avast is allegedly finding that particular "old" version.
So is it a Microsoft reporting issue? Or an avast analysis issue? --- since this just started happening yesterday I can't definitively place the blame here.
As noted above, today's definitions 13 06 18 - 0 have NOT changed/fixed anything.
Suffice it to say: We should do nothing now to "fix" IE. Just sit back. Relax. Take no action. And wait until the powers-that-be realize what's happening, and correct it.