If you look at the following section in the limits page:
https://docs.mongodb.org/manual/reference/limits/#Sharding-Existing-Collection-Data-Size

The table about max collection sizes reflects the right values.
However, we can't explain the numbers (256 and 400) in the following paragraph:

For existing collections that hold documents, MongoDB supports enabling sharding on any collections that contains less than 256 gigabytes of data. MongoDB may be able to shard collections with as many as 400 gigabytes depending on the distribution of document sizes.