We have built our own house and put in laminate flooring no problem. I can only think of two things that Persimmon might mean:
1. If it's a leasehold property, quite often the lease states no laminate, wood floors etc. even if you are in a house or ground floor as the lease might use standard wording that I think is more targetted at flats.
2. It does slow the build schedule down a little. It is better to let the house dry out more before putting laminate down and it is better to put laminate in before the architraves, doors and skirting boards are put in. Carpet can be whacked in at the end of the build the day before you move in, rather than holding workmen up.
Otherwise - they just want you to buy their carpets!