A few reasons I disagree with this analysis.
Based on Arena 6 data, my back of the envelope math suggests that for the ~2000 gems, you can get 3.3 magical chests, which should get you ~7.3 epics. Speeding the 80 chests for ~2000 gems should get you ~10 epics, plus ~6000 extra gold, plus ~6000 extra gold worth of of commons/rares. That's about 16 epic's value.
If you speed chests to get through the cycle, you don't actually have to gem open the magical chest. You can basically get the 12 hours for "free" overnight, so you can save gems that way vs. buying a magical chest.
If you are buying epics in the store, you are also at the whim of what is in there, which may be epics you don't want. Plus, you can only buy 8 per week at 2000 gold, so to get your 20 epics from 2000 gems, it will take weeks to obtain them. In the chests at least you are often getting 2x or 3x of the same epic. The store may or may not offer the same repetition (I don't know). So while it does appear that there is slightly better "value" for epics in the store, it is also at the expense of more time. To compare versus speeding, maybe you should also then add in the value of all the chests you would open "free" via the 2.5 week duration to buy in the store.
I think you are right though that this analysis is very dependent on how much you are spending. If you can't afford to gem open a full 240 chest cycle, then you are at the whim of where in the cycle you are today. At the very beginning, you won't make it to the magical chest with your 2000 gems. Later, though, you could speed through 2 perhaps. So it depends a lot on how much time you can "afford" to wait for your epics, and as usual, the cost of having them "now" is at a premium. I think that speeding chests does appear to present a pretty good value, however, even compared to the store.