Which web page are you referring to? Both Skyteam and Delta.com websites only mention 1 extra piece for STE and STE+.

Here is Skyteam website text --
If you’re an Elite Plus or Elite member, don’t forget there’s an extra allowance. It’s 10kg for Elite members and 20kg for Elite Plus, or one additional bag where the piece concept applies, depending on routing.

Note that just because Delta Gold Medallions get 2 free bags domestically does not necessarily mean all STE+ members get 2 free bags on domestic US flights.