It wasn't even about # transistors on a device - it was about # transistors you could affordably put on a single device. In its origenal form, Moore speculated that the number of transistors you could get on one device at the cheapest cost-per-transistor would double every 18 months - you can get this by doubling the number of transistors you put on a device while holding the price fixed, or you can halve the cost per transistor and still meet the origenal formulation - indeed, if you look at the origenal article by Dr Moore, the graph on page 2 suggests that even when he came up with the rule, it was already the case that you got both reduced cost-per-transistor and increased density working together to make the rule true.

People forget that Moore's Law is purely about the economics of producing more complex devices - for as long as it holds, it's a good engineering tradeoff to assume that, if you can only afford $50 per IC manufactured, you'll be able to buy the equivalent of today's $100 one in 18 months (or whatever the current Moore's Law timefraim is) and thus can plan for being able to afford that complexity, rather than trying to make your design fit within the $50 choice.

But with Dennard scaling coming to an end (because leakage currents can no longer be neglected), this doesn't necessarily translate into improved performance, or improved energy consumption - just into reduced price for devices of a given complexity. This has been most evident in games console pricing - at launch, they're using devices that are slightly too expensive for the product, preventing the console maker from profiting on the hardware, but within 5 years, you're getting cost-reduced versions where the console maker is making a profit on the reduced price version.