"I'd call a trick something that is not part of the documentation core .." I'd call this particular definition a pedantic, artificial nitpick invented soley for the purpose of picking nits in the post.
If on the other hand you use a conventional definition of trick, "a clever or particular way of doing something", then the post is indeed about a set of tricks.
But these aren't clever or particular ways of doing things. You're using Bootstrap's css classes to do exactly what they're designed to do.
The funny thing about your newly defined "trick" is that it would be foolish to rely on such undocumented behavior not changing on the next minor update. :)
Well the same applies to this definition of "trick" so I'm not sure that's a particularly damning point.
Either way, I didn't say such "tricks" are without peril - surely a number of jQuery "tricks," even those that were eventually canonized, have been broken over the years as the library updates.
I'd call a trick something that is not part of the documentation core, something that has unexpected effect or behavior that was unintended initially.
None of those would apply here.