How does the RTC of a STM32L0 keep the time accurately when using the LSI as the clock source?
The LSI cannot keep the time accurately.
Accurate timekeeping always requires a crystal.
All RTCs typically use a 32768 Hz crystal for that, be it a separate crystal or a crystal molded into same plastic package as a complete module.
The evaluation boards from ST are no exception to this, there usually is a crystal already mounted on boards or at least a place for mounting it if needed.
I am actually a bit surprised it is possible to select something else than LSE for RTC timing.