It seems highly likely that the context in the accompanying document will provide the best clue of DMR's meaning behind the quote. After all, this quote is just one line in the broad description of how Unix came to be, including its relationship with MIT CTSS.
Indeed, Unix and CTSS share many common traits, as DMR acknowledges in the document. But CTSS was a product of the 1960's, and was developed for the hardware at hand- which was IBM mainframe computers. Unix was a product of the 1970's, and was also developed for the hardware at hand. DMR goes on to make many references to the DEC PDP-11 in the document. This because, by the time of his writing, the Unix OS and the PDP-11 hardware had risen to influence in a symbiotic way. The whole document is providing context for Unix in relation to the computer hardware of this "new era".
"UNIX is a general-purpose, interactive time-sharing operating system primarily for the DEC PDP-11 series of computers, and recently for the Interdata 8/32."
Thus, it is very likely that the interpretation of "modern" in your quote simply means "for modern (contemporary) computer hardware". DMR had been witnessing the industry transition, as IBM mainframe hardware, originally designed for batch processing, gave way to minicomputers, more suited to interactive usage. Such "modern systems" benefited from a compatible OS (Unix) with "CTSS-like" abilities.
So, "modern" most likely just means an OS for the modern hardware, rather than for "aging" IBM mainframe hardware used for CTSS in the previous decade(s).
Note: The title of the document is "The Unix Time-Sharing System...". It is not a document about the Unix C code or even the OS design, as that is not a system. The word System implies an implementation of hardware and software, and this adds farther evidence that the word "modern" in the quote means for modern hardware.