A raft foundation is a thicker, substantially reinforced concrete floor from which the walls are built from. In most cases, this has a 65mm cement : sand screed on top. Whilst the concrete raft could conceivably crack during drying out this is highly unlikely. Indeed, it will have had the duration of the build construction (many months) to do so.
However, most likely cause of this crack is a crack in the floor screed which is quite common. It is usually caused by inadequate reinforcing and the fibre reinforcing often now used in mixes is often more susceptible. This thin screed will have dried out quickly.
I do not believe the line is caused by moisture but this could be re checked by taking up a small section to discount it. It is true that any following contractor should take all reasonable steps to ensure surfaces they work from are suitable and to the required quality before applying finishes. In this case, it is in my opinion, virtually unheard of for a flooring specialist to lay Amtico over an existing cracked screed.
Bloor Homes need to address the cause of the cracking and pay for your contractor to relay the flooring when it is sorted out.
The way Bloor Homes are fobbing you off is disgraceful.