LR parsers and ambiguous and non deterministic grammars
The grammar is really ambiguous. 

$\textbf{ if  } expr \textbf{ then   if }expr \textbf{ then  } stmt \textbf{ else  } stmt$

has two possible parses with that grammar. (The $ \textbf{ else }$ can attach to either $\textbf{ if }$.)

As you say, determinism and ambiguity are orthogonal. Certainly, there are unambiguous grammars which are not LR. But no ambiguous grammar is LR and this is one such example.