I guess it depends what you mean by a design evolution of the Evora rather than totally new. The current BMW 3 series, the G20, doesn't look all that different to the previous F30 generation and the two share some engines, but there is no question is is an all new car. The Emira shares the V6 with the Evora, but it has a different chassis, front and rear sub frames, all suspension components, steering rack, electrical system, HVAC etc, etc. It clearly is an entirely new car in terms of all of its major components except the engine, even if the external design has some similarities.
BTW according to the specs I have seen the Emira and Evora have identical wheelbases at 2575mm? Kinda feels like the Emira's should be shorter given the lack of rear seats, but apparently the rear bulkhead is further forward to allow for space and cooling needed for the I4 engine.