I think you are right. Sometimes fictitious planets are included in horoscopes, and this is one of the reasons why; some people continue to use it because some people included it in horoscopes in the past. There are others as well, such as the fictitious planets in the Hamburg astrology.
I have also read that apparently some astrologers decided to exclude Pluto from horoscopes because the IAU no longer considers Pluto to be a planet. That isn't a valid reason; Pluto still has coordinates and therefore can still be plotted on a horoscope (fictitious planets (such as Vulcan) also have coordinates although in that case the coordinates are made up rather than being real coordinates). Sun and Moon are not considered planets by IAU but are included on a horoscope too. You might decide to remove Pluto in case you think there are too many planets in the horoscope, but what the IAU says is irrelevant to horoscopes. (The Earth is usually the centre of measurement of a horoscope (although sometimes the Sun is used instead, but this is less common), so it is implicitly included in a horoscope but cannot be one of the objects plotted on it (since the plotting uses angular coordinates, and the object at the centre obviously has no angular coordinates).)
I have also read that apparently some astrologers decided to exclude Pluto from horoscopes because the IAU no longer considers Pluto to be a planet. That isn't a valid reason; Pluto still has coordinates and therefore can still be plotted on a horoscope (fictitious planets (such as Vulcan) also have coordinates although in that case the coordinates are made up rather than being real coordinates). Sun and Moon are not considered planets by IAU but are included on a horoscope too. You might decide to remove Pluto in case you think there are too many planets in the horoscope, but what the IAU says is irrelevant to horoscopes. (The Earth is usually the centre of measurement of a horoscope (although sometimes the Sun is used instead, but this is less common), so it is implicitly included in a horoscope but cannot be one of the objects plotted on it (since the plotting uses angular coordinates, and the object at the centre obviously has no angular coordinates).)