Accio is not supposed to work on people or creatures as per this source:

Why couldn’t Newt use ‘Accio’ to retrieve all his beasts?

‘Accio’ only works on inanimate objects. While people or creatures may be indirectly moved by ‘Accio-ing’ objects that they are wearing or holding, this carries all kinds of risks because of the likelihood of injury to the person or beast attached to an object travelling at close to the speed of light.

JK Rowling’s New Website

Yet in the new film Fantastic Beasts: The Crimes of Grindelwald Newt casts 'Accio' on a Niffler to get it back to him. He specifically says 'Accio Niffler'.

Doesn’t this break the rules set by JK?

Accio has been used on creatures before.

In the Harry Potter series, creatures have been successfully summoned by using Accio before. Harry successfully Summoned Neville’s toad Trevor.

“I’m almost certain of it,’ said Hermione grimly. ‘Watch your frog, it’s escaping.’

Harry pointed his wand at the bullfrog that had been hopping hopefully towards the other side of the table – ‘Accio!’ – and it zoomed gloomily back into his hand.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 18 (Dumbledore’s Army)}}

Also, Ted Tonks successfully Summons salmon out of a river.

“There ought to be a few salmon in here, or d’you reckon it’s too early in the season? Accio salmon!’

There were several distinct splashes and then the slapping sounds of fish against flesh.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 15 (The Goblin’s Revenge)}}

Therefore, the answer by JKR contradicts the book.

I'd argue that though JKR said that objects travel close to the speed of light on that website, that all of her writings in the books seem to contradict this, as she seems to describe objects as appearing to fly through the air, rather than instantaneously teleporting, which is how traveling near the speed of light a very short distance would appear to a person. Additionally, there are examples in the books of people using accio on living things, such as Harry accio'ing the toad he was practicing the silencing charm in. As late as book 7, "accio salmon" is successfully used, however, as the intent was to eat the salmon, there was no concern about killing the salmon with sudden g-forces. https://harrypotter.fandom.com/wiki/Summoning_Charm Therefore, I see that blog post as the contradiction, rather than the accio'ing of the niffler.