Has Cameron given up on Brown?

I mentioned below that Cameron was directly challenging Alan Johnson by coming out against PR.  Jane Merrick, the Political Editor of the Independent on Sunday, believes Cameron has given up on Brown and is now fighting Alan Johnson:

Johnson insists his increased activity in the past week is not a leadership bid. It doesn't need to be - it is enough that the cleanest member of the Cabinet sounds like a proponent of reform to the rotten Parliament. Cameron is rightly concerned about the threat the Health Secretary poses. It is not enough for Cameron to clear out the pension-age bedblockers in his party. The Tory leader has responded to Johnson's calls for PR by rejecting it, but coming up with more workable ideas.

Cameron has already written off Brown, and is now fighting the charismatic former postman.

She also makes the point that if Johnson did take over he would have to call an immediate general election:

Some ministers are daring to believe that Johnson could not only limit Labour's losses at an election, but even win - given that the new Labour leader would have to call an election immediately, so he would still be in his honeymoon period.

Was Cameron’s speech today really about constitutional reform or more to do with the threat posed by Alan Johnson?  I fancy the latter.