First: what is oncotic pressure?
Oncotic pressure, or colloid osmotic pressure, is a form of osmotic
pressure exerted by proteins, notably albumin, in a blood vessel's
plasma (blood/liquid) that usually tends to pull water into the
circulatory system. It is the opposing force to hydrostatic pressure.
"Colloid" can be any macromolecule. It can be albumin (the normal one). Immunoglobulins can increase blood oncotic pressure. Intact red blood cells do too and they can be important in that role. Blood substitutes using free and modified hemoglobin (loose in the blood not in RBC) also increase blood pressure by reducing fluid filtration across capillaries and keeping fluid in the blood vessels.
Thompson A et al. Stroma-free hemoglobin increases blood pressure and GFR in the hypotensive rat: role of nitric oxide. J Appl Physiol (1985). 1994 Nov;77(5):2348-54.
I think of it like rice in a salt shaker. The rice soaks up humidity because the humidity is attracted to and stays with the macromolecules in the rice. So too blood full of protein. The water likes to stay with the big molecules and so stays with the blood.
If you get too much macromolecule in the blood it can increase colloid pressure of blood and raise blood pressure and that can cause trouble. Maybe a professional bike rider has too high a hematocrit because he is abusing erythropoietin (that would be colloid pressure of whole blood, not just plasma). Maybe someone has a cancer like Waldenstroms that churns out loads of immunoglobulin (that would be colloid pressure of cell free blood, or plasma). High blood pressure is what you see in these cases. I suppose if you infused a rat with too much free hemoglobin blood substitute the same thing would happen.
People with high blood pressure still perfuse their organs. Filtration of fluid across capillary walls is decreased but the organs do ok. The converse situation is where people have problems: low colloid pressure allows fluid to freely leak across the capillary walls and people get edematous and puffy.
The situation you propose is a weird one. A person who has hemoglobin getting out of RBC has hemolysis: the red cells are leaking or blowing up. Common situations for hemolysis are malaria and sickle cell disease. Any increase in colloid blood pressure from the free hemoglobin is balanced by a decrease in colloid blood pressure because the red cells are gone. Congestive heart failure is not a problem with persons who have sickle cell crises or malaria unless they become super anemic which is rare and involves a different mechanism.
Not simple; sorry. But it would be easier to be simple if this actually happened (the free hemoglobin making heart work harder) and it does not.
No offense but maybe you have it wrong? Maybe take a photo of the text you reference and edit your question to add it (that little box with the mountains and moon will let you add a picture). Or type it in as you see it?