Portfolio Management -- short selling necessary, positive, selling, address, problem

liquidity

Schweser Book 5 Page 181: “To address the minimum-variance frontier instability problem, the analyst might consider constraining the portfolio weights (e.g. prohibiting short sales so that all portfolio weights are positive).”
Schweser Book 5 Page 212: Two key assumptions necessary to derive the CAPM are 1) investors can borrow and lend at the risk free rate and 2) unlimited short selling is allowed with full access to short sale proceeds. If these assumptions don’t hold, then the market portfolio might lie below the efficient frontier and the relationship between expected return and beta might not be linear.
So in one reading it says short sales lead to efficient frontier instability, but in another it says that unless short sales are allowed, the CAPM leads to inefficient results. Is there a discrepancy here or am I mixing apples and oranges?