I see what you are talking about. Any mention of a PT-1000 in the RTD-124 documentation must be a mistake. You may want to inform NI of the mistake.
I hope your all well.
Just to let you know, I just had a customer with the same issue.
There has been a CAR 46008 produced on the issue, it certainly does seem like an error in the operating manual.
The Spec Sheet shows RTD-122 module supports both 100 and 1000.
Best of Luck,