Maybe a more detailed explanation would help. Obviously I can't speak for all builders but I have seen quite a few.
FRP boats are very difficult to get out of a mold, many are damaged in the process and require some degree of cosmetic touch up. The split mold process I have seen is as follows. The gelcoat
is sprayed in first and then a skinout mat is applied. Some people call this a "veil" layer. The skinout mat is a short, random strand glass cloth and its only purpose is to hide the basket weave pattern of the heavier fabrics showing through the gelcoat
. It has no structural strength.
The above can be done with both halves of the mold already bolted together or it can be done with both halves separate. Some prefer to do it when the mold is separate because it is a finicky job to get the mat on the gelcoat and separated molds may make this easier. if they do it with separated molds, they may lay in some heavier cloth to stabilize the gelcoat and skinout mat. When the mold halves are put together many more layers of various glass fabrics are laid in with the number, type and placement dependent on the designers specifications.
The effect of this is a one piece structure. When the mold is separated there will be a "cosmetic" flaw i.e the mold "seam". The seam is the line where the mold was joined and most builders fill and fair this after the fact but this seam is often visible in production boats. I believe it is this often visible "seam" (in the cosmetic gelcoat and skinout mat) which causes people to believe that the two halves of the hull were bolted or glued together.