I for one have never said that Shi Potian is as good as Sweeper Monk or better than Xiao Feng or whatever. Frankly, I do not know how good Shi Potian is. I just know he is VERY GOOD, but how good...? Dunno...
Uh, we're talking about the two best martial artists of one era.But just killing two of the strongest martial artist of the era isn't really that big to a deal.
Can Xiao Feng defeat Sweeper Monk and Tianshan Tonglao combined? No
Can Yang Guo beat both Guo Jing and Golden Wheel Monk? No
Can Linghu Chong take on Dongfang Bubai and Abbot Fangzheng together? No
Can Yuan Chengzhi kill Mu Renqing and Priest Yuzhenzi at the same time? No
Can Chen Jialuo best Yuan Shixiao and Shaolin monk Tianhong? No
These are just examples to describe how exceptional Shi Potian is in the Jin Yong universe.
But how good are the two Lords of Gallant Island? We don't know. We only know that their two disciples Zhang San and Li Si are invincible at the Mainland, and during the era of Ode to Gallantry, only Zhang San and Li Si are known to have mastered the skill of "Sealing Acupoints without physical contact" (lingkong dianxue). In DGSD, only people of Emperor Duan Zhengming's level or higher are able to perform such a feat. Yet Zhang San and Li Si are just two out of many students of Gallant Island. So, let's say that the Two Lords of Gallant Island are good enough to be the teachers of Emperor Duan Zhengming of DGSD.
But the Two Lords could not even scratch Shi Potian.
You can draw your own conclusion.