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Both say “real PowerPoint”. Here is how to check.
GenPPT makes the same core promise Skiaw does, its homepage says it exports “true .pptx files you can edit anywhere”, in contrast to tools that lock you into their platform. That is the right thing to promise, and it is worth taking seriously rather than dismissing.
What neither a homepage nor this page can settle is how the file is built. So test it: export the same deck from both, open each in desktop PowerPoint, and check three things. Can you click a heading and retype it, or is the slide one flat image? Does the deck still look right on a machine that does not have the font installed, which tells you whether fonts travel inside the file? And do the boxes sit exactly where the preview showed them?
Skiaw's answers are yes, yes, and to within a single English Metric Unit, the geometry is measured off the rendered page rather than approximated, and TrueType fonts are embedded in the file. GenPPT does not publish its answers to the second and third questions, so we have marked them undocumented rather than guessing.