Is Minnan dialect the official language of the Tang Dynasty?

When Song Taizong was born, it was only twenty or thirty years from the end of the Tang Dynasty. Do you think Song Taizong, as a northerner at that time, could not understand the Mandarin of the Tang Dynasty?

The literati recorded an interesting story about Minnan dialect in Song Taizong. Song Taizong wanted to reuse Liu Changyan in Quanzhou, but his political opponents opposed it on the grounds that his dialect was incomprehensible. Taizong said, "Long words are not great but quality. If you judge a person by his appearance, you will lose your feathers. " Ministers said, "Long words are used suddenly, so as not to be overwhelmed by the times, or short words are difficult to understand in Fujian." Emperor Taizong added, "Only I can understand them." But after a while, the "meeting between the monarch and the minister" is limited. The holy family suddenly explained and said, I don't care about a word when I play a long speech. Then stop.

Did you get a look at him? Actually, I don't understand a word. What does this mean? This story, whether it is a scholar's joke or not, at least reflects the language situation at that time. This shows that there must be a huge gap between Min dialect and Mandarin in Tang Dynasty, otherwise Song Taizong can't understand it, and scholars can't treat it as a joke. Then the problem is coming. At this time, Tang Dou was extinct, and Min dialect was not the official language of the Tang Dynasty. How can it become the official language of the later Tang Dynasty?