Oldest association in America

Possibly. I admit I am not familiar with the history of the yearly meeting and don't know if it undertook any "associational" work or not.

My understanding is that the yearly meeting was of the Six-Principle Baptists, who were Generals.

Part of the preference in Baptist historiography may be that the Philadelphia Association was of Particulars, who were the mainstream of Baptist life (until recently), and Generals played a small part in the histories written by Benedict, et al.

(I would note that one of the beliefs of the Six-Principle Baptists apparently made an impression on their Philadelphia brethren, who adopted laying on of hands as an ordinance in their 1742 revision of the Second London Baptist Confession.)

The Philadelphia Association also probably had better publicity because of its aggressive missionary efforts and attempts - for lack of a better term - to be a court of judicial review to settle questions submitted by churches. The Philadelphia Association, it seems to me, to function as a denomination much as the General Baptist associations did in England.

The Philadelphia Association, it also should be noted, was the driving force behind the establishment of Rhode Island College (now Brown University).

BTW: The current Six-Principle Baptists say that the Rhode Island state flag's inclusion of the anchor and the word "Hope" (both from Hebrews 6) attest to the Six-Principle Baptist heritage of the state, though I cannot confirm this claim; the association's current logo also includes the anchor: