The more credible explanation is that the UK is not invading the embassy because Assange has some powerful counterthreat up his sleeve.
The US and UK are notorious for routinely violating territorial sovereignty and blatantly ignoring international laws and treaties, so the idea that a treaty would stop an embassy invasion is absurd.
Very few governments - even the powerful ones with nukes would break international treaties and send troops or police onto embassy grounds against the wishes of the embassy country.
"Groups with guitars are on the way out, Mr. Epstein"
Dick Rowe, A & R man
So now Ecuador will declare war on the US if it secures their embassy for an internationally wanted man... because that's the same thing as...