The 1k claim is about pricing
No it isn't. Read the quote.
A calendar may cost £250 but it’s actually "worth £1,000", they say, if you add up all the individual items.
which is covered by legislation.
What? I'm not aware that the government sets a minimum price for beauty products.
Worth is not a static value.
No indeed. Worth is about what somebody is prepared to pay for a product. The article even says
Sarah Johnson from Flourish Retail says that while there are many lovely bespoke calendars out there, "a lot [of companies] just use advent calendars to shift stock that may not be popular, rather than having to discount it"
It's a more palatable (to the companies) way of reducing prices on stock that won't otherwise sell. It's a discount that doesn't look like a discount as well as a marketing tool.