The extension does sound strong with the high-strength epoxy. I can't say what tolerances are required for an OD that particular lightweight stems are expected to clamp reliably onto, but I would expect that their design and testing is done on a steerer of consistent OD and straightness.
And if the OD of the epoxy extension were just a bit larger than the steerer, might the stem end up primarily gripping only the epoxy?
OK, I get that it's strong, but OD must be quite straight and consistent for best clamping and for realizing the tested strength of the stem.

Most stems grip a full 40mm of steerer length, and I know that there a lot of owners who would be happy to be able to add an extra spacer's height to their cut-off steerer.

Lastly I will add that craftsmanship counts, safe use of epoxy for structure relies on a good product of known strength and freshness.