specifically where it says that it produces "well over half the thrust".
If one looks throughout an engine, we get a bunch of pressure variation. Integrating over the various surfaces lets us express that in terms of normal forces. The observation is that, in a well designed installation, a large proportion of the nett forward thrust can be achieved from surface pressure contributions.
As to proportions, this will vary between installations.
The Concorde thread had some discussion on the subject so far as it relates to the Olympus installation and may be worth a read.
This page suggests that the Concorde had a prodigious proportion of non-engine thrust contribution in supersonic flight .. something in excess of 90%.
... standard engine installation design consideration, regardless of which OEM and model.