Yes, it's "converting" SCSS to CSS through the use of a formal grammar. In Computer Science this is called "compilation". So, I think you are getting confused in the terminology. It's not incorrect (though inaccurate) to call it converting, just know you need to understand that the people who wrote bootstrap are speaking a different language and computer scientists call the conversion of one language to another compiling, not converting.
Minification is an example of conversion. In reality, it's compression but you'd be more accurate calling minification conversion than you would with SCSS. SCSS -> CSS is certainly compilation by the strictest definition of the term.
If it simply said Sass compiling, I would be ok, but it said Bootstrap compiling which connotes more than just Sass and is the source of my confusion.