Searches for "pseudo mannequin" return many pages in French. Aside from that, it may be a made-up phrase or something specific to Amazon.
The 2009 version of the Category Style Guide: Clothing & Accessories states:
Do not use images where the product is shown with props, such as on a hanger, stand, or mannequin.
Both the 2014 and 2018 versions state:
Prohibited: Product images photographed on mannequin or digitally modified images
No mention of "pseudo mannequin", but even Amazon is confused about its policies regarding images that may or may not contain mannequins.
Ghost mannequins for photography are different from ghost mannequins for display; ours have removable bits to make the post-processing mannequin removal easier and tend to be much more expensive, but the garment trade version is mostly just a dress form that's really hard to stick pins into and will show at necklines, etc.
Those interested can read about 50 other types of mannequins.