Yes, it is true. An assortment of extra Pokemon, including several event Pokemon, were found inside Sword and Shield by dataminers when the games leaked last week. The 35 extra Pokemon are:
Bulbasaur, Ivysaur, Venusaur, Squirtle, Wartortle, Blastoise, Mewtwo, Mew, Celebi, Jirachi, Cobalion, Terrakion, Virizion, Reshiram, Zekrom, Kyurem, Keldeo, Rowlet, Dartrix, Decidueye, Litten, Torracat, Incineroar, Popplio, Brionne, Primarina, Cosmog, Cosmoem, Solgaleo, Lunala, Necrozma, Marshadow, Zeraora, Meltan, Melmetal
These Pokemon are excluded from the games' current PokeDex of 400 Pokemon, and exist only in the games' code. Currently, it is impossible to encounter them while playing Sword and Shield. However, their presence implies that plans exist for them to become available at some point in the future. It is possible this will coincide with the release of Pokemon Home next year, which may allow you to transfer these Pokemon to Sword and Shield.
This information comes from Kaphotics, who is the most credible dataminer in the community. It has been corroborated by other hackers, who have forced the game to create encounters for these Pokemon. For example, here is a video of Zekrom in the Wild Area. Obviously, you cannot replicate this without the technical know-how.