I thought the same exact thing, and got stuck for awhile because I thought the LSAT writers were trying to trick us by saying "fails to exclude" instead of "fails to include/acknowledge" or something

In terms of process of elimination, the answer for sufficient assumption will typically have stronger/more extreme language, whereas the answer for necessary assumption will be weaker. Also, sufficient assumption arguments will always have a language shift which can help guide you to the right answer. Language shifts are something new mentioned in the conclusion that wasn't mentioned elsewhere, and you need to find a "bridge" answer choice that connects that new idea to one of the previously mentioned premises. Necessary assumptions might have a language shift, but they don't always. Hope this helps