I think the author never passed a math exam.
Including forms it says. Pretty easy to get 100+ extra from forms considering they count stuff like Arceus's plates, Silvally's memories, Vivilon's patterns, Unown's letters, and Furfrou's haircuts as different forms.
The amount of forms varies depending where you count. As of USUM there are:
- 975 entries for stats and level up moves.
- 1080 entries for egg moves, plus 16 formes that don't have one (Pokémon that don't hatch directly, including evolutions of hatchable Pokémon, have empty entries, but for some reason cap Pikachus and SM totems don't have one, while USUM totems do). Strangely there are 20 entries for Scatterbug but only one for Spewpa, which suggests me originally Scatterbug was meant to have visible pattern differences.
- 1170 entries for models, plus 1 for the Egg model.
Model count is higher due to gender differences, most of them being just cosmetic and very subtle - the only really noticeable ones are Hippowdon, Frillish/Jellicent, Unfezant, and Pyroar. Plus Meowstic that is the only mon to have mechanically relevant gender differences, having separate movesets and abilities. Barring Meowstic, whether gender differences count as separate formes is up to debate.
Stat entries are fewer because many formes share them, like Unown, Deerling and Flabébé line (except Eternal Floette). Despite the type differences, Arceus and Silvally formes share a single stat entry, but they do have separate egg move entries. Interestingly Gastrodon formes have separate stat entries, which are identical except by color (which is a hidden stat, unused in battle mechanics). Totems have separate entries as well that only differ in size and weight, with the most relevant effect being Totem Araquanid and Kommo-o immune to Sky Drop, unlike their regular counterparts. (Kommo-o being ability-locked to Overcoat sucks tho, Araq takes better advantage of this perk)
I'd say the count in USUM would be 1077, which comes from the 1080 egg moves entries, plus the 16 formes without an egg move entry, minus the 19 unused Scatterbug entries.
Last but not least, there's the six Cosplay Pikachu formes, Spiky eared Pichu and LGPE starter Pikachu and Eevee for a total of 9 formes absent from USUM.