Does effective altruism only recommend things that are ‘proven’ to work?

No. However, we do think that we ought should consider evidence to be very important when considering where to donate. Some causes may be very speculative but because of the potentially extremely large number of beings that may be affected by that cause, now and in the future, it may be reasonable to give these causes serious consideration. We need to work on how to weigh things systematically as possible in our decisions. Fortunately, there are organizations working on exactly this, so we don’t have to go it alone.