Fun Fact:
In original designs, the "we say what is necessary and not what isn't" philosophy led at times to assuming players understand certain elements, which it later turned out they often didn't. The reason the vast majority of spells that you get from other features, in the original PHB, don't say that you can cast them with other spell slots, is because it wasn't initially deemed necessary to say this: the features generally say "You know the X spell" - and that statement alone, along with the spelcasting rules section, cover the fact that you can cast it using your spell slots. Many of those features also allow a free cast, without expending spell slots, which they state, but that was never intended to imply that you couldn't cast the spell normally - you 'know' the spell. That was the intention, anyway. When it hit player audiences large scale, it turned out that that wasn't how the vast majority read it... so the SA confirms that you are meant to be able to use your slots to cast spells you know, even if you gain them from other sources.
