The exact answer is probably unknown, but the Shield Pokedex entry for its Real form is: Leaving leftover black tea unattended is asking for this Pokémon to come along and pour itself into it, turning the tea into a new Polteageist. So this Pokemon enters a teapot, turning the tea into a Polteageist. Does this mean the tea turns itself into a Polteageist, or does the Polteageist simply turn larger? It's still not exactly certain.
However, judging by the image, it seems that whatever happens, Polteageist does indeed take up the entire teapot. Also, tea is never, ever purple and yellow, so it is definitely Polteageist. The tea Polteageist transforms is commonly described as black tea, and black tea is not that color. So probably, but we're not exactly sure.