On your site you state that widow's benefits do include the worker's delayed retirement credits, for example on this page:
But I called recently SSA, and asked their representative, and he said that survivor benefit does not include DRC.I could not find defined answer on SSA website, but some experimenting with their calculator https://www.ssa.gov/planners/retire/AnypiaApplet.html seems to confirm that survivor does not received DRC.
At the same time many financial web sites, besides yours, such as Kiplinger.com, thebalance.com state the opposite. So which is right?
The subject of the Q & A in the first reference you cite above was whether or not the spouse of a living worker would receive a higher benefit rate as a result of the worker's delayed retirement credits (DRC), and the answer is that they would not.
However, DRCs earned by a deceased worker do carry over to widow's and surviving divorced spouse's benefits payable on their record. For example, say John waited until age 70 to start drawing his Social Security retirement benefits and as a result received a monthly benefit rate of $2640, or 32% more than his full retirement age rate (PIA) of $2000. When John dies, his widow Jane could receive his full rate of $2640 as long as she is at least full retirement age when she starts drawing her widow's benefits.
What may be confusing you is that widows and surviving divorced spouses do not earn DRCs on their survivor benefits if they wait past FRA to start drawing. So, for example, if Mary qualifies for an unreduced widow's benefit of $2000 at FRA, DRCs would not be added to her widow's rate if she waited until age 70 to start drawing her widow's benefits.