Anyway, back to your point though. Why do you think Vanguard can offer lower ER's on some assets because they don't offer ETFs or funds for other assets?
Oh, that was just speculation that maybe they avoid offering funds in certain categories, asset classes, or tilts because they can't access them efficiently or in a way that would work for their customers. I think that's pretty clearly true in some of the more wacky asset classes like private equity or timber. Maybe it's true of some of the things mentioned in the tweet, or maybe not.
I picked microcap because it was first and took a total guess at some reasons off the top of my head - no idea if I was right or not. :)
Now that I look at the tweet more carefully, I notice he specifically mentioned ETFs rather than just funds. Which seems like a very fine distinction to make. Turns out that Vanguard has funds in two of the highlighted categories ("High Yield Bond" is VWEAX, and "Gold Miners" is VGPMX), but not ETFs.