بسم الله الرحمن الرحيم
The semen of both male and female is impure. However, there are two methods to purify it. If the sperm is thick and dry then it shall turn pure by scratching. But if it is wet or thin then it is necessary to wash it. The hadith which you have quoted speaks about scratching dry sperm to get purity:
قالت كنت … أفرك المنى من ثوب رسول الله صلى الله عليه وسلم اذا كان يابسا وإغسله إذا كان رطبا (آيار السنن)
So if any person offered salah while the sperm more than one dirham was smeared in his clothes then salah shall not be valid.
It is imperative to note that the lawfulness of scratching sperm is specific to the condition when it is thick, but since nowadays the semen is not so thick; hence it is necessary to wash it in any way. If the semen smeared in the clothes is equal to one dirham i.e. approximately 4.5 masha or less then salah shall be valid but makrooh. However, it is better to remove the semen first and then offer salah if it is smeared in the clothes. And if it is more than 4.5 masha then salah shall not be valid. We do not agree with the fatwa you have quoted.
Allah knows Best!
Darul Uloom Deoband