Yep, I'd agree with something along these lines.
In the instruction guide for applying for FSW-2, one of the criteria is clearly stated:
"Your current employer has made an offer to give you a permanent full-time job if your application is successful."
As I said, they might accept an updated copy without any issue. Or they might reject it based on the wording, as they were quite clear in what they asked for.
It's your call on this one. My advice would be to get the wording changed. Doesn't state a need to mention anything about temporary, just what they're offering you full time, permanent employment if the application is successful. I don't think your letter states that. In fact, your letter states you are permanent just now, which while in theory is true, does go against the rules of a TWP. How can you have a permanent job if your work permit if for a finite amount of time? When they issued your TWP, one of the things they generally ask/check is that you understand your job is not permanent based on your current WP, as you'd need to leave the country at the end of it (and any extensions). It's perfectly acceptable to apply for PR and intend to stay permanently, but it's still on the basis that's only if your application is successful, as otherwise you'd have to leave the country and your current job would therefore not be 'permanent' as circumstances outside the employer's control would mean you'd have to leave your job.
Sorry this isn't what you want to hear, but that's just our opinion based on what they asked for and their own guidelines. Perhaps there won't be any issue at all, but my advice would be to get it changed.
Graham.