When describing the witness field, reword "witness data" to "witness field" as "witness data" refers also to the marker and flag fields.
The confusion
https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki#transaction-size-calculations
Base transaction size is the size of the transaction serialised with the witness data stripped.
Here, "witness data" refers to having the marker, flag and witness fields stripped.
https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki#transaction-id
The
witnessis a serialization of all witness data of the transaction.
However, here "witness data" refers to just the witness field.