The reason for this belief is that currently the basic building block of Zen line is the CCX, which is 4 cores. Raven ridge is 1 CCX and GPU, Ryzen chips are 2 CCX, EPYC CPUs are made of 4x ryzen, for a total of 32 cores. But, when AMD advertised their server platform, the promised that in the next generation there will be 48-core chips that will be drop-in compatible with current server boards. Because the memory channels/PCIE lanes need to remain positioned for 4 chips, they can't just put 6 chips in. This implies that they will go from 8-core Ryzen to 12-core Ryzen 2, and the smallest change to make that happen is probably a 6-core CCX. And if they drop that same CCX into their 2nd generation Zen APU, that would make it a 6-core chip.
Our software still isn't really ready for it, but I guess that's to be expected.
It detoured into a company having no need to improve its products too much because they were already the best. If the technology is somewhat stagnating, it is best not to release all the improvements at once. After core counts have increased, what else will still make people buy new CPUs? Maybe I lack some imagination here - and so do companies making phone CPUs, which can't think of anything better than upping the core count.
Also, high core count CPUs are slightly more similar to GPUs, and Intel seems to be trying to keep the GPU threat at bay. There is no reason to believe that Intel couldn't design a competitive high end GPU, but that is not where its entrenched advantage is. (This part is not my own analysis, I read it somewhere else and consider it highly plausible.)