Japan's fight with Covid peaked much later than some other companies. But it probably exacerbated Polyphony Digital's already poor management. So I only half believe them.
Unless I'm mistaken, a great deal of production is now outsourced to India, so when you have teams that have to isolate while working on a game that requires real world references, especially on an audio front where Gran Turismo has particularly lacked compared to competitors, it makes perfect sense COVID impacted development. This is why I think 2021 will be a year where annualized racing games will be the only major new releases. F1, MotoGP, WRC, NASCAR, and Dakar are the only surefire releases this year. There's very little that's "standardized" in a Gran Turismo game, as there's no promise of particular cars or tracks compared to the other titles that focus on a specific calendar of racing events.
Unless you're into yearly sports games, this year is a backlog year.
Maybe a PS5 specific port of Assetto Corsa Competizione as the only game that isn't a series released every single year. Forza ain't making this year for much of the same reasons GT hasn't.