The i3 he recommended is Coffee Lake, newer generation than the i5 Kaby Lake. The previous i3 processors were 2 core, 4 thread, the new i3 is a 4-core, 4-thread. Given some minor tweaks, and the slightly larger cache memory, it should perform on par or maybe slightly better than the older i5.
The different motherboard is to support the Coffee Lake generation of processor.