IMHA is an immune process where RBCs are destroyed by abnormal production of antibodies against the organisms own RBC. The body can't produce them fast enough to keep up with the destruction.
The leukemia/lymphoma is a cancer where there is an overproduction of abnormal cells (usually lymphocytes) and that production "takes over" the bone marrow so there isn't normal production of RBCs. The RBCs aren't being destroyed, just not being produced.
They can both present with similar symptoms - anemia, lethargy, inappetence...