It happened the almost exact thing to you:
i didn't have enough score, so I was invited by BC to apply for their EE stream and got 600 points. I also thought that I was invited through CEC, so I only submited a LOE that I was applying under that stream, hence a POF was not required, and just reviewed today my ITA and it says PNP and not CEC.
Apparently you don't need to be CEC only, it might even apply for other programs, the only prerequisite is that you have a valid job offer, plus a valid work permit, according to:
http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=1145&top=29
"The system currently asks all applicants to provide a proof of funds document. If you don’t need to provide proof of funds, you must upload a letter explaining either that you have been invited to apply under the Canadian Experience Class,
or that you have a valid job offer."
I am assuming that if you were invited by Alberta, then you do have a valid job offer, so I guess there is nothing to worry.