Embassies are not the land of the foreign country. Embassies are considered sacrosanct from uninvited local LEOs, because if you violate their embassy, you've created a precedent where anyone can violate yours.
While true, the technicality of the case means that Swedish prosecutors want to interview a Australia person on UK land inside a Ecuadorian embassy.
But since Embassies are considered sacrosanct, it is the Ecuadorians that exercised their right to dictate the rules of the interview rather than the three other involved nations.
In the Assange case I recall even the prosecutor saying that the kind of things was common when doing police work over borders, but that they hoped that things would be done on time. From what I remembered, it was also mostly on time.