To quote the other relevant rules passage for context (page 17):
The lowest you can have an attribute is 1, and the highest is 10.
There is no explicit explanation of how this interacts with the Kiai spending option you have quoted, and so I would be inclined to think it does not interact in any special way; that is, the minimum and maximum values for an attribute still apply.
That said, though you did not ask this, it is possible to raise attributes above 10 through temporary means (e.g., as a Samurai, Kongōki, etc.).
Hey I Can Chan noted that one of the sample adventures ("Lotus Blossom's Bridal Path") contains a character ("Flying Razor") who has a base Attribute exceeding 10. This character happens to be an NPC, which the rules explicitly state are flexible when being created by the GM as compared to a PC. Notably, written on page 154 is:
Heroes, paragons and others will have higher attributes, including 10 or more. Major enemies may have attributes ranked up to 20!
This character is main antagonist for that adventure, and as far as I can tell is meant to be fought by the entire party at once. (Note also that he gains additional Vitality per character in the party, a feature generally reserved for important boss characters.) When fighting him, the characters are expected to be making use of their Kiai in order to win rolls; if the PCs could improve their base stats far enough, the contests would be less concerning, which I think would harm the drama of that confrontation.
In general, I think the 10 Kiai cost to raise attributes balances it enough that this is unlikely to be an issue in play (spend Kiai now to win rolls regardless of attribute, or raise attributes for long-term gains), but your experience will probably vary depending on the particular game.