I think that Syndra feels that Lux isn't a threat and is far weaker than her. Afterall, Syndra allows her magic to be used freely as if it were water. Lux is a bit more controlled, something that Syndra would likely scoff.
On the flipside, Lux wouldn't underestimate her boasts. Syndra is a powerful mage by all standards. If Lux and Syndra did fight, chances are she would lose. At least the first time.
After that, Syndra's full power approach won't be as effective because Lux would have already learned how to counter it, or even emulate it herself to a lesser degree. If there is one thing Syndra lacks, it's adaptability. Lux seems to be a bit of a master at adapting to the powers of other mages, or even using their own abilities against them, as in the case of Morganna. If Lux isn't finished off in the first fight outside the League, I have full confidence in her abilities to defeat Syndra should their paths cross again.