To quote Wikipedia:
On March 9, 2015, the 13-inch model was updated with Intel Broadwell processors, Iris 6100 graphics, faster flash storage, increased battery life, and a Force Touch trackpad. On May 19, 2015, the 15-inch model was also updated with similarly fast flash storage, increased battery life, the Force Touch trackpad, and an AMD Radeon R9 discrete graphics card on the higher-end model. The higher-end 15-inch model also added support for dual-cable output to 5120×2880 displays. The 15-inch models were released with the same Intel Haswell processors and Iris Pro graphics as the 2014 models due to a delay in shipment of newer Broadwell quad-core processors. Apple continued to sell the 2015 15-inch model until July 2018.
Source. So yes, depending on how generously you define it, they are "basically same machine" to some extend. Besides faster CPUs, GPUs and flash storage and somewhat improved battery life, the main improvement was the inclusion of the fantastic Force Touch trackpad which is a pretty big plus in my book. But outside of these changes they are pretty much the same, yes.