[dagw]

"The only plaza" is either unambiguous (in cities with one plaza) or incorrect (in cities with multiple plazas). Just because a language makes it impossible to be unambiguous doesn't mean it makes it impossible to be wrong.

[noglorp]

perhaps, you might say 'the assumed plaza' for explicit-ambiguity?

[Deestan]

Ah, yes. That is actually what the Lojban sentence would have said. My "translation" of that part was a bit off.