Presumably they mean that acceptance of migration is a continuum and the numbers of climate refugees would push a large fraction of people self identifying as liberal to oppose it.
What makes it a different question? What numbers change that change things?
Leaving aside what you hear on the internet, I personally know a few people who say they have no concern welcoming any number of refugees, a couple of them actually to the point of getting quite enraged at people who do have different opinions. So there definitely are some such people in existence.
No liberal would want millions of people coming from Afghanistan, for example.