It's just something that I've often read on mental illness forums, they are usually knowledgeable about the pharmacology of psych meds. It makes sense because the minimum dosage for psychosis/mania is 400mg and the effect of antipsychotics that helps psychosis/mania is supposed to be dopamine blockade. Lower doses are used for anxiety, sleep and depression.
ETA: I haven't made much effort to search but on the wikipedia page for quetiapine it says "At very low doses, quetiapine acts primarily as a histamine receptor blocker (antihistamine) and
α1-adrenergic blocker. When the dose is increased, quetiapine activates the adrenergic system and binds strongly to serotonin receptors and
autoreceptors. A
t high doses, quetiapine starts blocking significant amounts of dopamine receptors.
[49][60] Off-label prescriptions, e.g. for chronic insomnia, of low-dose quetiapine is not recommended due to the harmful side-effects.
[61] "