If you read this on r/zen then ignore it. Every other poster there don’t know what they are talking about it and try to assert inaccurate and false claims upon Sōtō Zen due to their personal dislike of it rather than actual historical basis.

Other than that he did not invent Zazen, instructions can be found in earlier Chinese texts like “Mozhao Ming”, “Zuochan Yi”, “Dialogues of Hongzhi Zhengjue”, etc.

Some modern scholars question how much contact Dogen actually had with his claimed teacher Tiantong Rujing due to differences in their teaching and the way Dogen’s narrative changed over his life and through history as later followers also changed it. He is said to have emphasised different throughout his life.

As for whether it’s really Zen or not... I’m not sure if that’s a historical question but very few people would deny that it’s Zen. It follows the characteristic “Sudden” approach attributed to Zen, the Shobogenzo by Dogen though written in a different style contains classic Zen teachings like Mind is Buddha, etc and the practice of Shikantaza is a modified version of the sitting meditation practiced in China before the radical reforms of Dahui Zonghao.

If you are interested I would recommend reading “Did Dogen Really Go To China: What he wrote and when he wrote it”

Here’s a essay by the same author that discusses this topic.

http://www.thezensite.com/ZenEssays/DogenStudies/Did_Dogen_Go_to_China.html