But the 8086 was used in microcomputers, not mainframes or minis.

The broader interpretation of this idea, that the x86 era saw a shift away from vertically integrated computer manufacturing, is absolutely true. The narrower interpretation, that the 8086 chip triggered a shift away from vertically integrated computer manufacturing, is not.

The "x86 era" is really the era of microcomputer architectures eating mainframes and minis. I suspect that would still have happened if the 68k (or Z8000, NS32000, etc) had won the war instead of the x86.