Reading the verse (translation is NKJV, take from it what you will)
"as also in all [Paul's] epistles speaking in them of these things in which are some things hard to understand, which untaught and unstable people twist to their own destruction, as they do also the rest of the Scriptures."
It seems to me, rather than explicitly saying Paul's epistles are scripture, that Peter says that Paul's writings are a part of the faith that the apostles teach which includes Scripture (that is, the Old Testament).
Though, in sorting out which epistles are to be considered canon, this endorsement of Paul by a direct apostle of Christ must have been important in including him in the canon.
I don't see a strong case for sola scriptura here; I actually see the opposite.
It would be fun to quote the second half of the verse to double-predestinationists.