You're right with a strict reading. "In general, dependent types ..." should be a statement about all instances of "dependent types" or (weakening to one common English usage) a statement about typical instances of "dependent types". In neither case, are they undecidable.
A loose reading (probably overly loose, but possibly what was intended rather than what was expressed) would permit something closer to "Dependent types, in full generality, are undecidable to type check." which I believe to be the case, where actual implementations are more constrained (though decreasingly so!).
A loose reading (probably overly loose, but possibly what was intended rather than what was expressed) would permit something closer to "Dependent types, in full generality, are undecidable to type check." which I believe to be the case, where actual implementations are more constrained (though decreasingly so!).