Find out if an attribute is set with setCustomAttribute() or not

I am really pulling out my hair. I have a function to return all product attributes in my Magento 2 Module:

public function getAllProductAttributes(): array
    {
        $attributes = [];
        $collection = $this->attributeCollectionFactory->create();

        foreach ($collection as $attribute) {
            $attributes[$attribute->getAttributeCode()] = $attribute;
        }

        return $attributes;
    }

MagentoCatalogModelResourceModelProductAttributeCollectionFactory is the $attributeCollectionFactory.

I have not been able to 100% determine if an attribute is considered “custom” or “normal”, meaning if I have to set it with setCustomAttribute() or setData().

“is_user_defined” doesn’t work, since for example the product “description” is set by setCustomAttribute() but is not user defined. I tried multiple combinations of attribute parameters, however nothing worked.

Anyone have a solution?