My understanding of the passage you are alluding to is Marx viewed religion as palliative care. Religion was the opioid of the masses in the sense that people can’t or wont be given real medication. The patient is dying and nothing can stop that, so at least make sure they don’t suffer. The role of religion was to minimize suffering and would fade away when suffering was gone.

Also even if he had not thought that way he is not the be all end all on the subject.