GPL 3 vs Proprietary LiveCode

[Paul Looney]

In another thread Richard Gaskin wrote:

"And as a dual-licensed system, LiveCode lets us choose either GPL or other licenses depending on our goals for the project at hand.”

How can you legally have a “dual-licensed system” that uses GPL 3 code, when the GNU General Public License Version 3 states specifically that it does not permit incorporating GPL programs into proprietary programs?

Is not “Commercial”, which is approximately 95% “Community”, also subject to the terms of the GPL 3, and therefore open source?

Sincerely,
Paul Looney