Well, in some cases it's scam, but its genuinely the case that power at different times of the day has different value, and most NEM agreements completely ignore the cost of transmission, which itself is quite hefty.

So it is plenty reasonable that you wouldn't get 1:1, especially if the grid is already able to satisfy all demand during peak sunlight by using just base load + solar. Some power companies turn it into a scam anyway and set grossly disadvantaged prices for consumers, but just because it's not 1:1 doesn't mean that it is a scam.

The energy market has many similarities to the stock market. People also bet on the prices going down or up by contracting themselves to produce a set amount of energy at a given time for a given price. There are companies which only do trades and has no own production, earning their profits only through market predictions.

Well, infrastructure costs money.

But talking about the cost of transmission sure does highlight issues with this billing model. Because if it's going to the neighbor there's negligible transmission. The engineer's argument was very stupid.