So are we saying this happens? On a pre-turbo (or draw through) system:
1. On full boost at WOT, MAF reads X amount of air. Engine adds X amount of fuel.
2. You come off the gas completely. TB shuts closed.
3. Compressed air has nowhere to go, so pushes back into the turbo.
4. Air goes through the turbo back past the MAF into atmos pressure, causing the MAF to read air again.
5. As the MAF is reading air movement, the engine adds X amount of fuel despite no extra air going past the TB.
That's the only way I can see a non-BOV system adding fuel after you close the TB. On a system with a VTA BOV that sits after the MAF, then yes that would cause the engine to run rich as you've dumped metered air (as has already been said).