I presume the EU has proper rules about reporting 'attendance', but North American sports leagues/entertainment venues are very good at 'selling out' by selling their tickets in a block to ticket resalers. So the race probably was sold out, from the point of view of COTA organizers, just that TicketMaster or StubHub or whoever had piles of unsold seats.
The EU does have attendance rules. However, their rules allow any event which sold all tickets available for the sets existing at the time of the performance, and that had a defined maximum number of permitted tickets (e.g. it had a rule about how many people could be put in the general admission area and that many tickets were sold), to describe itself as "sold out", even if the number of seats was reduced, the price of tickets was reduced, some seats never got constructed (and the tickets for them never put on sale or subsequently withdrawn) or (not mentioned in my previous post) some of the tickets were sold for free/given away (a common thing at certain levels of sports, including most non-F1 British motorsport, is to supply free tickets to children and the carers of people with disabilities. The "sellout" rules even allow venues to send batches of free tickets to schools, for example, on a proactive basis and still use the tag). It can also still be used even if none of the people who bought tickets turns up (provided the reason was outside the control of the organiser e.g. weather).
Selling in blocks is allowed, but when doing so there are rules about checking all those tickets were sold on (as hotels would do), used directly (e.g. school tickets) or returned (the latter, if no subsequent sale/distribution occurs, rules out using "sell-out" to describe the event once the returns come in, though there is allowance for adverts that went to print in the gap but aren't published until after returns begin to arrive). Failure to monitor this prevents the use of "sold-out". (I think the EU requires any group of above 9 tickets to be treated as a block, but some countries may use a lower threshold and the UK currently requires monitoring of any group of more than 6 tickets).
The American system sounds like it allows even more ways for a sellout crowd to be, well, not sold-out.