The way these things work is that the media processor supplier, Sigma Designs, supplies a evaluation kit that includes a reference design for the customer to play around with. They get all the tools (compilers, linkers, etc.), proprietary software libraries, Gerber files for the PC board, parts lists, etc to recreate the reference design themselves. Very often the customer's product ends up being based on the reference design with very few changes. It used to be that many of the graphics board manufacturers that used NVIDIA graphics processors just based their boards on an unmodified NVIDIA reference design.
So I'm guessing that they could tell very early on if there were any bugs in the Sigma EM8621 since they had the eval board, and given that there are many products shipping on the EM8620 and working reasonably well, what you described is not what's going on. It's not even their processor in the first place. And the vendor was able to supply the platform to evaluate the processor when TViX first conceived of the product, so it's doubtful they are debugging the chip for Sigma.
I'm not even going to speculate what might be the cause for their delay, as I know little about the company, or how much of previous products' software they might be able to reuse, etc. But I really doubt it's what you described, because that just isn't how these kinds of products are developed. Ok, I will speculate that their delays are due to features they are implementing that are not part of the Sigma reference design.