The voting threshold issue has nothing to groups outside the P5. It's 0nly about voting "within the P5" and not "the voting body," as you wrote.
You're right in a sense, but you're also mistaken. This is from CBS explaining why they want the voting structure changed:
"The NCAA Governance Steering Committee's current proposal says that for the Power 5 conferences to create new legislation they would need approval from two-thirds of the 65 Power 5 schools and the 15-athlete voting bloc plus four of the five major conferences. The Power 5 conferences, including the SEC, want a 60-percent threshold among themselves and athletes and three of the five conferences. They also want the ability to interpret new rules passed."
They're not worried about how the votes are differentiated among them, it's that within the legislative body as it currently stands, they don't have enough votes by themselves to get the autonomy they're looking for. They're basically trying to get enough autonomy to have their own voting authority that can trump that of general Division I legislation, and they don't have it.
Say a Division I rule proposal is on the table, and it's going to pass Division I as a whole. The power five conferences want to be able to override the legislation if they have a 60 percent voting bloc. But to do that, they have to get enough votes to change the structure. That's what they're lobbying for in the interim.
Ultimately this is all about autonomy. They are just fighting for a voting threshold where they can self-govern with 60 percent threshold being met.
This is confirmed with the following Slive mention.
'Slive raised concerns that the current proposed voting threshold won't be high enough to pass legislation, such as cost of attendance, medical care and issues related to agents. Slive said he is optimistic the governance structure will pass.'