The shape of an obelisk has a narrow, constant width. The shape of a pyramid has a width that is wide at the bottom, and narrow at the top. The 'y axis' of the shape is how far along in the startup life cycle the company is. The width is how easy it is to get funding at that point in the cycle.

Here's how I interpret it:

It used to be that the ease of funding was relatively constant at all points on the life cycle. Now it's easier to get funding at an early stage, and harder to get funding at a later stage.

It's not an idiom I've heard before but I think what PG means is that getting Series A has always been difficult and that's the top point of either an obelisk or pyramid. The lower rungs of funding (angel) just have a lot more activity than they used to, so the bottom is wider like a pyramid (as opposed to an obelisk which is narrow... as in the old days).

There's a much wider base of companies that get initial funding, but not much more that get later stage funding.