While I don't necessarily think it absolutely necessary for these mons to have second types, I do find it odd that some Pokemon seem to have "honorary" typings without actually being that type.
There are quite a few Pokemon who have attributes of a certain type but are often single-typed or whatnot. Not necessarily the like of the oft-discussed Charizard and Gyarados, but there are other cases that I'm thinking of. Like Gohtitelle, Goodra, Klinklang, etc. Gothitelle for example learns several Dark-type moves, some by level-up especially, and its design and Pokedex appearance come off as a Pokemon that could conceivably be part Dark-type. Klinklang learns several Electric-type moves but is pure Steel. Goodra is notoriously pure Dragon-type amongst the pseudo-legendaries, but it evolves from Sliggoo only when it's raining, learns numerous Water-type moves in its level-up pool (including Rain Dance), and has Hydration as one of its abilities. It's also notably native to wetland climates. Grapploct is another example: it's pure Fighting-type, but like Goodra it also has several Water-type attributes, learns Water-type moves, and is native to the ocean. Florges is also an honorary Grass-type in the sense that it has several Grass-type moves and has numerous attributes associated with Grass-types. All of the mentioned examples are single-typed, but could conceivably be dual typed (aka Psychic / Dark Gothitelle, Dragon / Water Goodra, Fighting / Water Grapploct, Fairy / Grass Florges, Steel / Electric Klinklang, and vice versa).
I dunno, I find it really sticks out that several single-typed Pokemon in particular have that honorary second type in the form of their movepools and attributes, but don't actually *have* that type. I wonder what exactly went into the design process for that kind of Pokemon, and whether there's a design standpoint explanation for those instances or if there's an in-universe reason they would only be single-typed and not have a second type. There are probably other examples out there, but these stick out the most to me. Of course, there are also Pokemon out there who could conceivably have three types (Fire-type Salamence, Poison/Dark/Bug Drapion, Grass/Ghost/Flying Decidueye), but that's a bit of a lesser example.