I would prefer them saying the evidence pointed that way to them using a stronger phrase.

But that's not what that means - it's not very strong. If I say, "My search proved fruitful", then I'm not saying anything particularly strong - just that I found something. Saying "that proved unlikely to work" just means "based on <whatever work I've done>, I've observed that it's unlikely to work". <whatever work I've done> can be a search, some research, an experiment, or anything of that sort.

Note that this sense of "proved" does not even need to imply a particular conclusion - "The experiment proved inconclusive".

This is more similar to the use of "proof" in baking or alcohol than the use of "proof" in geometry or logic.