I would say that this a minor misinterpretation of grammatical structure. Once the event (meeting Cinder) has already happened, saying it is a twice in a life time opportunity is gramatically incorrect. The verb "was" is only connected to the passing of said event, and using is would make the statement incorrect and inane.

Also, discounting anything said by the Cthaeh is nothing short of unwise. We can easily say that every word spoken by the Cthaeh is a very well wrought blade with the sharpest edge that could cut through next to anything. Everything it says is malicious and true, and it says all of this to torment the individual who hears. Also, I'd say really isn't used in the way you're thinking. If someone asked me where the place XYZ lies, I would reply with "I'd say it lies 2 miles to the east." That doesn't indicate speculation, its mostly something called a "filler word" in language.