This has never been a serious question. People have looked at DOS for years and never found any stolen CP/M code. Tim Paterson has written about how DOS was designed here: https://dosmandrivel.blogspot.com/2007/09/design-of-dos.html https://dosmandrivel.blogspot.com/2007/08/is-dos-rip-off-of-.... His employer was selling an 8086 computer but had no operating system for it, so he proposed making QDOS, a placeholder operating system that they could sell until CP/M-86 was available. He made QDOS compatible with the publicly documented CP/M API, but did not look at how the API was implemented. This made it possible for software companies to easily make QDOS ports of their existing CP/M software by automatically translating the source code from 8080 assembly to 8086 assembly. Paterson himself took advantage of this. The 8086 assembler and source code translator were originally CP/M programs written in 8080 assembly, then he translated them to 8086 in order to make QDOS self-hosting.

It's somewhat ironic that Kildall was angry about DOS copying the CP/M API because Digital Research went on to release DR-DOS, an 8086 operating system that was API compatible with MS-DOS.