Clarification on assignment

I am dealing with civil lawsuit at district level. Correct me if i'm wrong here. I understand that in specific 3rd party debt collectors who do not buy debts (non junk debt buyers) claim they are legally allowed to enforce legal means with an assignment contract with the original creditor. Under RCW 4.08.080.BUT!...Here's my question, if i'm affirming defenses against their legal right on a claim/STANDING on a lawsuit under the premise that they are NOT the real party of interest by name under Federal Court Civil Procedures 17(a)(1) and RCW 34.05.530 requiring ALL 3 prongs to be met to have standing in my case in particular prong #2.Here's the key part i'm trying to make sure i'm reading correctly, If i'm asserting defenses that they have no stake in the game because they lack standing AND they didn't buy the debt to own it, Owners rights to the debt... Does their assignment become non enforceable ?, Here is part (B) of RCW 4.08.080 :''PROVIDED, That any debtor may plead in defense as many defenses, counterclaims and offsets, whether they be such as have heretofore been denominated legal or equitable, or both, if held by him against the ORIGINAL OWNER, against the debt assigned, save that no counterclaim or offset shall be pleaded against negotiable paper assigned before due, and where the holder thereof has PURCHASED the same in good faith and for value, and IS THE OWNER of all interest therein.''