That would still effectively be 5 years of updates which is a hell of a lot better than what most manufacturers currently offer.

Sure it would be, but I think the implied point above was that including the period the device is sold (what's that? say three years), the total support time is much closer to the ten years that was being discussed up top, and pumping out more new devices may shorten that back to closer to five years.

Does having 3 devices that you are supporting for 10 years make more work than 10 devices you support for 5?

Because while that doesn't make sense in my head, I could easily see that being the methodology used by the Android device manufacturers.

If these ten devices are mere SKU updates, then maybe yes, they create a sliding five year support window (plus however long they're being sold), instead of a ten year one. Keeping software working for a five year old hardware platform sounds a lot easier than doing that for a ten year old one. A lot less Git branches also.

If the ten devices are mere SKU updates made solely for the reason of getting around the regulations, then the company can expect a rather huge fine :^)

Of course, but any increase is an upgrade to the current situation.