Did Saber ever learn Eastern fencing styles before meeting Shiro?
In the Fate/SN world, heroes learn everything they need to know about about age they were summoned in. Knowledge such as how to operate common machinery, customs, and language are automatically learned during their summoning. As an example, Saber learned how to ride motorcycles in Fate/Zero. Motorcycles did not exist in her age, but she knows how to operate one nevertheless. This is also the reason why there is no language barrier between Saber and Shirou.

As Kendo is a common sport in Japan and a martial art involving swords, Saber could have learned this particular Eastern fencing style as part of the Japanese customs she had to learn from her summoning.