No, it IS a JS developer issue.
Constraints like this exists in virtually all languages. C/C++ compilers have a limited number of cases in a switch operator, and have limits on template instantiation depth. MySQL cannot join more than 32 tables. There are limits everywhere. If some feature works in a clean room environment, it does not mean it will work seamlessly in a production environment. Everything should be used in a way it is supposed to be used. And semicolon operator should be used to separate statements, not comma.
Opera now is in Microsoft's hat, because when ms made new versoion of windows (say, Windows 95, 98 ...), they had a lot of hacks to support buggy software written for previous verions. It was error in a third-party software, but it was Microsoft's responsibility to make a workaround. Because it's they who will be blamed.
So, when twitter is not working, it's Opera who will be blamed by end-users, that's why they are striving to fix it (and cursing twitter devs at the same time, i suppose)