It's to circumvent the fact that Star Alliance Golds no longer get a free bag at all on all TP tickets (all Economy Discount tickets come without any free baggage allowance, and with no entitlement for *G to check a bag for free).
Therefore, to avoid confusion, as long as you have a ticket that involves any other Star carrier - which will have at least 1 bag for free for *G - then TP will also give the same allowance, even if the *G is travelling on TAP Discount Economy and therefore not entitled to a bag on that sector.
However, the text you highlight above is NOT on the page you have linked to. Can you provide the link to the page?