These are the tweets he was banned for:

> The 75,000,000 great American Patriots who voted for me, AMERICA FIRST, and MAKE AMERICA GREAT AGAIN, will have a GIANT VOICE long into the future. They will not be disrespected or treated unfairly in any way, shape or form!!!

> To all of those who have asked, I will not be going to the Inauguration on January 20th.

Twitter said the first tweet "is being interpreted as further indication that President Trump does not plan to facilitate an 'orderly transition'" and the second is "being received by a number of his supporters as further confirmation that the election was not legitimate".

So they banned him because they wanted to not because of TOS violations. If you can interpret "I will not attend" as "It's illegitimate" you can interpret anything as anything and ban anyone for any TOS provision.