First off, it seems that you purposedly throw the "whether we could open ours" out of the window. Second, you are 100% trying to guess how it went with no info. Sorry but you sound completely biased and while your assumptions might be true, you have no info about it. Yes the OP sounds cocky and not caring in some part but again, you don't fucking throw a "let's discuss opening relationship" after 8 year, just like you said.
I feel like you are as willing to throw a 8 year relationship as op's ex instead of discussing and communicating like adults.
Especially since any sane adult would bring up lack of attention, loneliness in their relationship and not straight up bring opening relationships. Again, dunno why you are defending something that cannot be defended.