After trying it myself, I have to say that you're right; I haven't been able to see Boulder Dash as a Skillbook either, even after I tried switching its link from SkillbookEarth to SkillbookFire.

Theoretically, it should be working. Boulder Dash has its own SkillData entry (with Skillbook name), there is a RootTemplate for a skillbook that points to the correct skill, and the Link file correctly links the GUID of the RootTemplate to the SkillData. And yet it alone does not appear to work.

The Localization entries do not appear to be the issue; adding the Skill directly to my character at Creation (through tweaks to the mod) enabled me to use the correct skill. The Boulder Bash books did not say "You already have learned this skill" when I browsed Arhu's stock with Boulder Dash in my skillbar.

I honestly don't know why Boulder Dash appears to be the exception; it uses the same patterns that every other skillbook use. I suppose I'll have to make a psuedo version of Boulder Dash as well.