It looks like libre calc is not reading the formulas embedded in the cells of .slk file
(SYLK may be slightly outdated,)
How did you get to your conclusion? What origin are your SYLK files of?
Just tried it with a .slk created by LibO itself. Formulas were imported correctly and worked.
However, the SYLK format will only handle one sheet, won’t it?
Can you provide an example demonstrating your problem?