The Barbie star is 35 and blonde, while the original Catherine Earnshaw in Emily Brontë's 1847 novel was brunette and a teenager, leading to some criticism before the flick is even out.
Reacting, Robbie told British Vogue: "I get it".
"There's nothing else to go off at this point until people see the movie," she said.
Elordi stars as Heathcliff, who is "dark-skinned", so his casting also raised eyebrows, but there's a whole line of actors who have played him before that did not meet that description.
Robbie was blown away by the 28-year-old actor's portrayal, hailing him "our generation's Daniel Day-Lewis".
Insisting cinema-goers need to give the movie a chance, she said: "I saw him play Heathcliff.
"And he is Heathcliff. I'd say, just wait. Trust me, you'll be happy. It's a character that has this lineage of other great actors who've played him, from Laurence Olivier to Richard Burton and Ralph Fiennes to Tom Hardy.
"To be a part of that is special. He's incredible and I believe in him so much. I honestly think he's our generation's Daniel Day-Lewis."
Fennell herself has defended her casting of Robbie, saying she has the "supersized star power" required to take on such a feisty female.
"Cathy is a star. She's willful, mean, a recreational sadist, a provocateur. She engages in cruelty in a way that is disturbing and fascinating," she said.
"It was about finding someone who you would forgive in spite of yourself, someone who literally everyone in the world would understand why you love her."
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