Yes, at the end people are just calling the same type of product with the name of the most famous producer of that product, right?Personally, I don't expect most casual people to know the difference between the engines anyway, so it doesn't really bother me people would call Kafu, Teto, etc a "Vocaloid." Even though I know they are not, I won't go and correct people.
For how I see it, it's not that different from calling every type of tissue "Scottex"