Is tenure also meant for becoming an assistant professor?
Japan's academic system is still closer to the continental system where there is only full Professor (教授） in a department and several assistant professors (助教授）and a mixture of adjunct and full-time readers/lecturers　(講師）. The assistant professors can be tenured.

Some schools in Japan are moving to the American system where there can be several full professors in a department and there are now some colleges that are introducing the category of associate professor　(准教授）). 

Japanese labor laws tend to protect full-time faculty, so you can assume that you have lifetime employment if you pass the first employment review -- if and only if you haven't been hired as a part-time (非常勤) or otherwise term-contract employee (非正規雇用).

In this case, it sounds like they want to emphasize that you are NOT being hired as part-time  or other non-tenure track (such as the new "jokyo" 助教 which is not the same as a tenure-track assistant professor 助教授).