The palace must have been approached by journalists about the matter and probably knew the story would eventually leak anyway. When I read the story in El Pais to yesterday evening I immediately thought of Prince Joachim. I am sure other royalty-watchers and royalty-journalists must have had the same thought. And as lula stated, it was already rumored in Spain. The palace was right to get ahead of the story.
It's interesting that
El País only talked about 'a Belgian aristocrat'. It further explains that is was a family reunion to celebrate the arrival of one of the boyfriend of one of the family members.
According El País, the 14 days quarantine rule only applies to tourists, so if Joachim entered Spain for other reasons (work? I thought they lived and worked in South Africa - I guess Victoria's family was able to come up with an 'internship', so he would have a reason to travel - other than visiting his girlfriend), this rule apparently would not apply. Diario Córdoba does not make that distinction in their reporting and states that anyone traveling from abroad needs to self-quarantine for 14 days.
I fully understand that Joachim and Victoria wanted to be together after spending most likely months apart from each other; although they should have waited if it was against the rules for him to travel to Spain (but having a diplomatic passport does wonders). Nonetheless, even if it was legal to travel - it was very unwise (and if all at the same time not allowed) to already meet with that many people 2 days after arrival.
I wonder where he picked it up: during his travels on Sunday would be a reasonable possibility.
Diario Córdoba adds that 5 people at the welcome party were from Sevilla and at first they had not been able to contact them (but later on the Distrito Sanitario de Sevilla was able to track them down).
So, it seems that on Sunday Joachim flew to Madrid and took the AVE to Córdoba.
On Tuesday they had a welcome party with (mostly?) family members at Victoria's parents' house in Córdoba attended by 27 people.
On Wednesday Joachim started to develop symptoms
On Friday (or Thursday - reports vary) he was tested for Covid-19 and found positive (on Friday); that same day they tracked down the 27 people and all of them were put in quarantine.
Also on Friday: they started their investigation into whether any prosecutable offense took place (the fine could be between 600 to 10.000 Euros).