I'm wondering, if the EDC such as bisphenol A, was chlorinated during the water treatment process, is the toxicity of formed disinfection by-products (DBPs) from EDCs smaller than the EDCs itself?

Although often underappreciated, BPA is structurally similar to thyronines and can bind to thyroid hormone receptors.  Chlorination (and halogenation in general) improves that relationship.

Bisphenol A are inherently toxic and can induce endocrine disrupting effect. Therefore, bisphenol A free plastic items are required by regulators in many countries. Depending on the treatment type, it is safe to imply that degradation products of Bisphenol A are generally non-toxic or less toxic than parent compound. However, the water matrix and its composition play an important role in the formation of toxic DBPs. 

I once wrote a paper on Bisphenol A, the degradation products were studied after photolysis, toxicity remains an issue to be answered.
 DOI:  10.1080/02652030600889541 

Chlorination works for  removing dangerous bacteria - mainly Enterobacteria.  Without  it many  humans and domestic animals die.  Flushing human  hormones down the toilet is a different sort of problem  that is not sorted out by chlorination.

BPA is an environmental estrogen and have significant affinity for estrogen receptors

Depends on several factors. If BPA is not decomposed, some of the mono, di, tri chlorinated BPAs even tightly bound to the receptors and act as secondary estrogens.