The Swedish Embassy counts as Swedish soil, legally. He was to be 'detain Mr Assange for questioning on suspicion or rape'. That is suspicion, not charging him.
sigh, you're repeating that untrue claim about extradition for *just* questioning.
But anyway, in addition to what others have said about whether an Embassy counts as the country's soil, let's examine this possibility from a practical POV.
If indeed the Embassy is exactly like the country, then the Swedes should be able to detain JA following questioning, while they either fact check his statements or decide to prosecute (after all, if this happened in Sweden, do you think JA'd get bail?). Wouldn't such detention be viewed as illegal confinement by the UK and/or Ecuador. If so, this'd be a clear criminal act occurring on embassy grounds and the 1987 act would allow the UK to enter. It'd be hilarious if Ecuador started asking for the UK to enter the Swedish embassy in that case
Then, should they decide to prosecute, they could have the entire court in the Embassy and if convicted and sentenced to detention, detain him there. Hm, that doesn't sound right.
Further, if it is Swedish soil, the US could presumably request extradition from Sweden the moment he enters, and as extradition from Sweden is apparently so easy, it would be granted. Now to get him from Swedish embassy to the US would require the permission of the UK, which is exactly the permission that would be needed to get him from actual Sweden itself, if he were extradited there. So, being interviewed in the Swedish embassy would not give him the protection he claims he needs under the claim that being interviewed in that embassy is equivalent to actually going to Sweden.