You need to know enough in the technology you are designing for to be able to estimate the effort for developing each of the alternative designs you might consider.
As a UX designer, you are the person who negotiates with the users the way their interface will look like. The users will always push for what they imagine will fit their needs, plus eye candy. The developers will always push for a information structure and navigation which is as close to their OO class architecture as possible (or whatever technology is used in the background). You are the one who has to be aware of all the trade-offs. You have to know what happens when you concede a feature to the users. Does it mean one more line of source code, one more day of implementation, or does it require the whole planned architecture to be rearranged? Or do they simply want magic? If you knew how often I have heard from the user, "And one more small thing - this search we are talking about, it should of course be able to search in [three other unrelated systems] too, and display the result within our system".
You will have to deny features to the users, sometimes because they are a bad need fit, sometimes because implementing them is so hard it would break the budget. You have to be able to do this accurately, and also to explain the reasoning to the users. You have to come up with alternatives which address the needs as far as possible, while assuaging the users' largest fears and ensuring that they are technically feasible to implement. So, in order to be able to spot a good compromise, you need to be able to evaluate the costs of each alternative in terms of work the implementation team will have to invest. You don't have to be able to implement it yourself, but it sure helps having tried a few simple implementations in the target technology, to know the basics, so you can at least have the right feeling of the impact of each alternative under consideration.
And while we are at "tried a few simple implementations", don't fall into the trap of thinking that the effort needed to implement a feature in prototype-grade software is the same as the effort needed in production-grade software. There was an excellent post by Raymond Chen about what it would take to change a simple message shown by Windows, sadly I can't find it right now, maybe somebody can supply a link.
You might be able to do your work with less technology understanding, but this is the level you need to become a good UX designer.