Joanna might have been too busy, or at least I hope that was the reason. Because one other possibility could have been that Bill realized that he would quickly be out of his depth in a three-way conversation about a Marvel movie if the other two were Joanna and Van.
I listened, it was fine, but it would have been better if Joanna, Mal, Chris, or Sean had joined them. But because they weren't there, it became less about the birth of the MCU and more about the rebirth of RDJ's career, which is clearly what Bill cared about in this film. Which he made even clearer in his patented absurd/lookist/sexist recasting suggestion: he wanted Leslie Bibb's Vanity Fair reporter character, who has maybe three lines, replaced by Scarlett Johansson. When Van raised that would have meant that ScarJo couldn't play Black Widow in the MCU movies that would follow, Bill dismissed his objection by stating that, unlike Van, he didn't care about those movies.