Fake Hadith of Eclipse?

Q: The following hadith which was a sign of truthfulness of the Hadrat Mirza Ghulam Ahmad(as), the Imam Mahdi and Promised Messiah, has been graded dhaif so how is it a sign? 

حدثنا أبو سعيد الإصطخري ، ثنا محمد بن عبد الله بن نوفل ، ثنا عبيد بن يعيش ، ثنا يونس بن بكير عن عمرو بن شمر عن جابر عن محمد بن علي قال : إن لمهدينا آيتين لم تكونا منذ خلق السماوات والأرض ينكسف القمر لأول ليلة من رمضان وتنكسف الشمس في النصف منه ولم تكونا منذ خلق الله السموات والأرض 

“For Our Mahdi, there are two Signs which have never appeared before since the creation of the heavens and the earth, namely, the moon will be eclipsed on the first night of Ramadhan (i.e, on the first of the nights on which a lunar eclipse can occur) and the sun will be eclipsed on the middle day (i.e on the middle one of the days on which a solar eclipse can occur), and these signs have not appeared since God Created the heavens and the earth.” (Sunan Darul Qutni)

A:There is a hadith in Kitab al-Fitan of Nu'aym ibn Hammad (d. 228 AH) which literally predicts da'ish. Now, from an isnad point of view this hadith is very weak (da'if jiddan) and has at least three weak points ('ilal). But now there are many contemporary scholars who say that this hadith is authentic (ma'anahu sahih), because it was fulfilled, despite the weakness of its isnad.

For example: https://www.youtube.com/watch?v=10XXszA1dXw

This can also be held against our oppenents by us for the hadith on the solar and lunar eclipse! 

Look at the double standards of the non-Muslim scholars. When it comes to the hadith of the Mahdi by ad-Daraqutni, they say: واعلم أن الحكم بصحة الحديث أو ضعفه إنما يتم بالنظر لسنده لا بالنظر للواقع كما ذكر أهل المصطلح "And know that the hukm on the sihhah of a hadith or its du'f can only be completed by looking at its sanad, not by looking at reality/what happened, like the terminologists say."http://fatwa.islamweb.net/fatwa/index.php?page=showfatwa&Option=FatwaId&Id=55108 

But when it comes to the hadith on da'ish, the very same website says the following: لو ورد حديث ضعيف يخبر عن وقوع شيء ما ووقع كما ورد في هذا الحديث فهذه قرينة يتقوى بها الحديث "If there is a da'if hadith about something to happen and that thing happens like it was said in that hadith, then this is an evidence that stengthens this hadith." http://fatwa.islamweb.net/fatwa/index.php?page=showfatwa&Option=FatwaId&Id=15838