These guys are confused, they're lying to themselves.
Honesty in the context of industrial design takes into account the materials, manufacturing and physical form.
And in graphic design, respecting the limitations of print.
Pixels are intangible. The 'honest' that's respected within industrial/graphic design, fields that have influenced this new 'flat' digital style do not translate due to the fact that pixels are intangible. The ideals that defined the modernism movement were based on the tangible.
What if a "non-flat" design improved communication/interaction? E.g. I touch this button therefore using 3D to communicate such interaction would make sense.
The designers that promote these ideals are confused.
Honesty for me in this context comes down to the HTML/CSS/JS crafted to create these visual and interaction elements.