On a motion to add a party to a tortfeasor on the basis of discoverability, does the discoverability principle apply?

On this motion the proposed defendants accept that the plaintiff did not have actual knowledge of the identity of the tortfeasors he now wishes to add but claim that the plaintiff has not established on the evidence before me that he has acted with such due diligence as to justify adding them on the basis of discoverability. Wong v. Adler was a clear case of the plaintiff having actual knowledge of the third parties' identities and it was not necessary to consider due diligence. As indicated in the quoted passage from Wong v. Adler: "If the issue is due diligence rather than actual knowledge, this is much more likely to involve issues of credibility requiring a trial or summary judgment motion, provided of course that the plaintiff gives a reasonable explanation on proper evidence as to why such information was not obtainable with due diligence." I left open a window however to challenge the addition of a defendant based on due diligence "if the evidence is clear and uncontradicted that the plaintiff could have obtained the requisite information with due diligence such that there is no issue of fact or credibility". It will be rare that the applicability of the discoverability principle based on due diligence will be determined on a motion to add a party. The third parties claim however this is exactly such a "clear and uncontradicted" case.