It could be enforced for the majority of people just by blocking the download of non-backdoored software from well known sources. And then for the relatively few tech-literate types who still obtain and use E2EE messaging software, the government will have a ready way to prosecute you whenever you do anything inconvenient, or look like you might do something. So it can be a serious problem even if it can't technically be enforced for everyone.

The law is enforceable. If the options you're given is "put a backdoor in your product or stop operating in the country", it'll happen. And even if you reply "then I'll go away", laws like this, stupid, dangerous, breaking everything, will keep popping in one country after another until it's too late.

It not making sense have no bearing on whether it can be enforced or not. And the mere existence of the law may be enough to later put you in hot water if you have some de-facto illegal software on your phone or computer, for example. It would not be automatic everywhere, but another tool to just legally have something against most people.

That's being too generous. Some may not fully understand, but many do and simply don't care. Not sure if better or worse, but its not entirely lack of understanding.