The thing is, the lords of Xia Ke Dao were considerably better than the other fighters on the island(like Zhang 3 and Li 4). While fighters on the island were ridiculously more powerful than everyong else. So SPT was literally god-like in terms of ability relative to everyone else. DFBB, despite living up to his 'invincible' nickname, was not able to separate himself in such a way.It might be interesting to note that Laviathan used the example of SPT defeating both the Lords of Xiake Island, who were the next two best in the world, as an example of why he thinks SPT is very strong, and yet that argument doesn't seem to apply for DFBB.
One can make a good argument for DFBB to rank anywhere from Xiao Long Nu(who was definitely a notch below the top fighters in ROCH), all the way to being the equal of the top DGSD fighters. There's just not enough evidence to say either way. Shi Po Tian is similar in this regards. Both just don't have enough comparable opponents to truly test them.