I wonder what the point is of them marketing a product on their site of which the entire inventory is already sold. Are they after a halo effect?
At 500 cars, I wonder if the sales even pay for the project. Then again, they get the aforementioned halo effect AND they get tech to trickle down into future lower-end vehicles.
500 cars * $1million each = $500 million in revenue. From previous disclosures I would estimate R&D costs for this car are $50-80 million, production costs ~$200 million, and a large chunk for sales and marketing. They also have to provide owners with high quality post-sale services such as event invitations, free track days, complementary services, etc.
But after those expenses they are easily making $150 million profit on this. Hypercar sales are a cash cow if you can convince rich people to buy your cars.
The 'halo effect' you mention is also quite true, as these hypercars increase brand value and get the slightly-less rich people to buy their lower end models.
However, quite often they make a design change, call it a different model, and do another limited production run. McLaren did exactly this with their '675LT' model. The initial production run was 500 cars which proved very popular and sold out the day they were announced. However there was still much demand for the car. So McLaren made a convertible model of the car and named it the '675LT Spider' and did another limited production run of 500 cars.
This behavior is not limited to McLaren. Bugatti is very guilty of this, they've ran at least 4 different 'limited production runs' of their Veyron.
This product is so exclusive, there's no way you can even think about having one!