Depending on where you are with your CF that's not actually true.If you can do some aerobic I think that would be more beneficial for PFTs than weight lifting.
Thanks for your infoWeight training with enough intensity will illicit more airway clearance than cardio. Slow steady state such as jogging or walking will make you cough at the end, but not really during as much as weights. Make sure you use enough weight to create an oxygen deficit - that will create amazing airway clearance.
When it comes to cardio - interval training is best for airway clearance. For example, 60 seconds at 50% of your max heart rate followed by 60 seconds of 75% of your max heart rate. Repeat this for 20 minutes and you'll have amazing airway clearance.