Wow I finally got it - seems that asking the question out loud helps to find the answer !
Together with the missing test of PDL_INTERRUPT_ENABLE_RTC0 in interrupts_fm0p_type_3.c, there is also some errors in rtc.c file in RTCInitNvic function which enables TIM_IRQn (IRQ 14) instead of WC_RTC_DT_IRQn (IRQ 15)
Hope it helps anyone trying to make RTC interrupts working on the S6E1C3 !
To conclude, here is a summary of the changes I've made to make it work :
/* Changes to make RTC interrupts working : rtc.c, lines 254 - 256 NVIC_ClearPendingIRQ(WC_RTC_DT_IRQn); NVIC_EnableIRQ(WC_RTC_DT_IRQn); NVIC_SetPriority(WC_RTC_DT_IRQn, PDL_IRQ_LEVEL_DT_RTC_WC); rtc.c lines 286 - 288 NVIC_ClearPendingIRQ(WC_RTC_DT_IRQn); NVIC_DisableIRQ(WC_RTC_DT_IRQn); NVIC_SetPriority(WC_RTC_DT_IRQn, PDL_DEFAULT_INTERRUPT_LEVEL); interrupts_fm0p_type_3.c, lines 1053 - 1056 #if (PDL_ON == PDL_INTERRUPT_ENABLE_WC) || \ (PDL_ON == PDL_INTERRUPT_ENABLE_RTC) || \ (PDL_ON == PDL_INTERRUPT_ENABLE_RTC0) || \ (PDL_ON == PDL_INTERRUPT_ENABLE_DT0) interrupts_fm0p_type_3.c, lines 1083 - 1084 #if (PDL_ON == PDL_INTERRUPT_ENABLE_RTC) || \ (PDL_ON == PDL_INTERRUPT_ENABLE_RTC0) */
Unfortunately, I think it is to be done for every code generation...
I will bring this the notice of our PDL team.