The school's guidance on SOP doesn't match to what I know about it. What should I do?
I assume this is for a PhD or research-oriented Master's program.  Almost certainly the admissions committee will care about your past research.  Here are some possible reasons why they don't explicitly mention it in their description of the SOP:


They don't assume that you've already done research.
Someone wrote it without much thought or experience.
Presumably any past research would play a role in your desire for a further degree, so it is implicitly covered.
They were just expecting this information as part of your research statement.


Also, regarding your comment "This contradicts to what I know on SOP": different departments (and different committee members on the same committee) look for different things, so this place may not look at community service at all.  Answers on this site are not infallible universal laws.