Including similar language in their agreement would probably have made the EU's already terrible case even worse anyway, since it would almost certainly have only covered AstraZeneca's non-UK European facilities, and the whole problem seems to be that those can't produce nearly enough vaccine to cover EU demand. Note that the current clause about the initial supply of doses only covers manufacturing them within the EU; the part about the EU including the UK explicitly does not apply to that clause. (Indeed, the only mention of UK manufacturing seems to be exempting it from a clause banning the use of non-EU manufacturing sites.)
Somehow I doubt the UK would have been so happy to rely on in-EU production of the Pfizer/BionTech vaccine if they'd done that. As I understand it, the UK production facilities for the AZ vaccine exist mostly because the government wanted at least one UK-produced vaccine in case Trump pulled some kind of vaccine nationalism move and restricted exports. If there was reason to suspect the EU was the one likely to do that...
