Holding back the 100-400 for the 200-400 1.4x doesn't really make sense to me because the latter is so much more expensive.... except if the new 100-400 costs 5000$ upwards. But then again it wouldn't really replace the old model. So something about this sounds fishy if you ask me.
It seems valid to me to first sort out production issues and satisfy customers that have ordered a lens that has been officially announced over putting effort into producing a lens that hasn't yet officially seen the light of day.
5D3, 5D2, Sony α6000, G16 | EF: SY14/2.8, V20/3.5, 28/2.8 IS, Ʃ35/1.4, 50/1.8, 50/1.8 STM, Ʃ50/1.4 EX, 100/2.8L IS Macro, 16-35/4L IS, 24-105/4L IS, 70-200/2.8L IS II, 1.4x II, 2.0x III, T28-300 VC PZD, 70-300L IS, Ʃ150-600 OS HSM S | E: SY12/2, 35/1.8 OSS, 16-70 ZA OSS, 55-210 OSS, SY300/6.3 ED UMC