This follows the pattern of "Juma ni jina la mtanzania. " Why doesn't it mean "Rehema is the name of the kenyan"?
That should also be accepted. I'd say jina la Kikenya for "Kenyan name"
I would also say "jina la Kikenya." Unfortunately, the actual sentence is false or misleading, since there is nothing particularly "Kenyan" about the name Rehema. A much better example would have been, "Rehema is a woman's name." (jina la mwanamke OR jina la kike). Likewise for Juma. There is nothing particularly Tanzanian about the name Juma. Much better would have been "Juma is a man's name." (jina la mwanamume or jina la kiume).