how can you have a bishop when you don't yet have a church?

how could Papias be an early christian bishop if early christianity misunderstood the text and didn't actually take off as a religious sect until after bar Cochba?

Papias dates from around 60 (just before the destruction of the temple) to around 155 (well after the bar Cochba revolt).

According to tradition LXX was translated into Greek under Ptolemy II (ca. 285 BCE to ca. 283 BCE), which means Greeks, Greco-Egyptians and Greco-Romans had access to the text from that time.

The Pauline letters are presumed to have been written ca 40-50 CE.  This means that the greco-Roman communities which understood yehoshua hamoshiach to be a name were in contact with a Judean who was attempting to assist them in becoming acceptable to the Judean Temple community from that time on.

Papias was born into that era, in the part of the world where Paul's communities were located.  It is reasonable to assume he joined one of those communities if he was not born to parents who were already members o one.

Papias was exposed to and involved in communities that had already evolved their understanding (based on transliteration and mis-translation of text).  Because he was around at the time of the bar Cochba revolt, when the "gospels" were written to try to assimilate the revolt into the Judean canon, Papias would have been among those who would have heard of their existence, and of their substance.  Simply because those communities did not look like what we think of as a "church" does not mean that those communities had not orgainzed themselves into groups that had their own leadership.