Assuming that the allegation that the poster who was promoting mt-card was really using an email used to advertise for r4i deluxe gold, that is pretty damning evidence that the people behind MT-Card and r4i deluxe gold are the same and are not being very upfront with their customers. However, that in no way really links them to 3dslink and orange3ds.
The speculation behind that, as far as I know, is based solely on the fact that their carts are identical apart from the stickers on the carts. This is more of circumstantial evidence. It could be possible that the manufacturing of the carts was outsourced and the contractor sold overruns or simply accepted orders for the exact same cards to other groups.
I don't really see any reason for r4i to sell the exact same product under different brands unless they were perhaps selectively selling their other brands at lower prices to certain segments of the market who would be unwilling to buy for any higher price. So far there is no evidence supporting this kind of theory so I am inclined to believe that 3dslink and orange3ds are separate groups who somehow managed to procure identical carts either from an industrial subcontractor or possibly at a discount for a bulk purchase from r4i themselves on an agreement that they would sell them under a different brand name.