Apparently, only the DS1074Z-S has the waveform generator.
You can find the same bad verbiage in Amazon's listing on the DS1074Z too,
The DS1000Z Series is the new economic level Digital Oscilloscope from Rigol to meet the customer's applications with its innovative technology, industry leading specifications, powerful trigger functions and broad analysis capabilities. The DS1000Z 4 channel oscilloscopes come in 70 or 100 MHz versions with a 7 inch display and Rigol's UltraVision technology as well as a host of options. Add the optional analysis, decoding, deep memory, and integrated 2 channel waveform generator for a powerful 4 channel scope at an exceptional price.
That verbiage seems to be copied from the Rigol site on the ds1000z series which is in itself wrong. However the ds1000z series includes DS1054z, DS1074Z, DS1074Z-S, DS1104Z and the DS1104Z-S. With only the
-S models, (the DS1054Z-S and DS1074Z-S) having
Built in 2 channels 25MHz waveform Generator(DS1000Z-S) listed under their features.