Oh I see, that's odd isn't it! We took the insert out of ours, it didn't say anything about it. Maybe you could email them with your problem and see what they say??
Yeah I'd go with that. On all 3 car seats we have (all different brands) they all say the same thing, infant insert to only be used when rearward facing, but you can remove it after a certain age/weight.

Eden is nearly 7 months and still rearward facing. Her seat goes to 12kg rear facing so we will be waiting until that time. DD1 was turned at 13/14 months. DS was turned at around 7/8 months because I didn't know any better