You are not including the negation ni to your translation. Siquiera alone can mean "at least", but ni siquiera is always closer to "not even." If you loosen up the translation so that it is not so literal ni siquiera could potentially be translated as "at least", but it would be taking liberties. For example:
- Ni siquiera nos saludaron is best translated as "They didn't even say hello to us", but informally that could be "They could have at least said "Hi" to us".