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 

Bisphenol A (BPA) and its chlorinated derivatives have an impact on various endocrine glands:

1. Thyroid Glands: These substances can affect the production of thyroid hormones, crucial for regulating metabolic activity. Some studies suggest that chlorinated BPA may have a stronger effect on thyroid hormone (T3) than BPA itself.

2. Adrenal Glands: These glands, responsible for stress hormone production like adrenaline, may also be influenced.

3. Sex Glands: The system producing sex hormones, including testosterone and estrogen, may be affected, leading to issues such as infertility and sexual health problems.

4. Pituitary Gland: This gland, regulating the production of various hormones, could also be susceptible to disruption.

Both BPA and its chlorinated derivatives act as artificial estrogens, interacting with estrogen receptors α and β. These interactions can lead to cellular changes, potentially contributing to cancer development through increased cell proliferation, apoptosis, or migration. Consequently, consuming BPA and its chlorinated derivatives may lead to endocrine disruptions, heightening the risk of cancer, obesity, and diabetes-related diseases.

The extent of endocrine effects of BPA compared to its chlorinated derivatives remains unclear and requires further research to determine which effect is more pronounced. Additionally, chlorination may not mitigate their harmful effects but could lead to the production of more toxic by-products, negatively impacting human health.

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.