FRom what you have described they are correct.
Your maternity leave does not break your continuity of service, but the period which you are on maternity leave doesnt count towards your LSL entitlement period.

So if youve been there 8 years, take 12 months mat leave, this doesnt mean your years of service start again from zero when you go back to work, but the leave doesnt count as year 9 either.
Youd go back after the twelve months off and still have 2 years to go to get to your 10 years.