The Sovereign explanation fits much better within the MCU continuity following Guardians of the Galaxy Vol. 2’s release.
As time passed, they found a way to enhance the cocoons, eventually leading to the one that Adam Warlock will one day come out of.
With Guardians of the Galaxy Vol. 2 having been out for almost two months, James Gunn provided the above response when asked on Twitter if the cocoon seen in the first Guardians of the Galaxy movie contained Jean Grey’s body when the Phoenix took over.
Since Ayesha, the Sovereign’s High Priestess, called Adam a new member of their species, it makes sense that their ancestors could have come out of cocoons like the one The Collector acquired.