It is true his speech was so archaic and formal that average people couldn’t understand it, but as I answered in a previous thread, it was not because his everyday language was any different from theirs, but because the manuscript prepared by the Cabinet was written in kanbun kundoku-tai, a literary language only used in highly formal documents. As making a kanbun kundoku-tai sentence requires special knowledge of classical Chinese and Japanese literature, the manuscript was drafted by Hisatsune Sakomizu, the Chief Secretary of the Cabinet, and edited by Mizuho Kawada and Masahiro Yasuoka, scholars of Chinese literature close to the Cabinet, and the Emperor just read it out as requested. If you listen to the recording carefully, you’ll notice the Emperor himself was struggling to read the text, with awkward pauses found at 1:33, 2:04, 2:22, 3:32, 3:43 and 4:04, for instance.

Kanbun kundoku-tai is a variant style of Classical Japanese whose sentences are so constructed as to look as if automatically translated from Classical Chinese texts. Classical Japanese is a literary language based on Early Middle Japanese, the language both spoken and written in the Heian period, which spanned from the 9th to 12th centuries. In the 11th century, the colloquial language began to evolve into Late Middle Japanese, while the literary language remained as it was, at which point, it can be said, colloquial Japanese diverged from Classical Japanese.