The recommendations refer to dedicated memory; however shared/dynamic memory can tend to work just as well. Dedicated memory will always be faster as the card itself can manage all the memory that is used when running an application like DirectRT. Shared/dynamic memory will be slightly slower and may impact performance.
Also - I've lowered the minimum specs. down to 32 MB's of memory. 128 MB's would be -far- more than needed (not that there's anything wrong with that).
To answer your question -
Yes; a laptop using shared/dynamic memory would be capable of running Direct RT.