Because it's a statement about the potential of a trade deal. Historically, some trade deals have yielded such benefits, and the size of the economic territory involved means that the potential upside here is significant.

But they don't know the potential of this trade deal because they do not have the text of it so cannot draw a comparison with historic trade deals. To present something as positive based on history when the thing is secret is nonsense. Is like someone hovering around a shell game who only talks about the previous winners.

It's easy to assess the potential of the trade deal, by the simple expedient of counting up the various deadweight losses attributable to tariffs. It's not like we don't know what existing tariffs are or what costs they impose on trade flows, that information is public. For a very rough estimate of the potential gains, you can look at the size of the market for a given trade area and multiply that by the measured increase in economic growth from previous free trade agreements, such as NAFTA, the EU and others.

If you want to learn more about it I suggest this CRS primer: http://www.fas.org/sgp/crs/row/R40502.pdf