That was a very clever line and an important part of the "soft reboot" aspect of TMP.According the the novelization of STAR TREK: THE MOTION PICTURE, James Kirk flat out stated that the missions set in "TOS" were exaggerated on purpose for public consumption.
And really, it worked fine that way through the movies and most of TNG - any references to TOS were always made through the lens of the current product - Khan sported a metallic SF chevron, the TMP Enterprise graphic was used to represent the TOS-E, Kirk got name dropped, Sarek appeared in person etc.
The audience was free to visually reimagine (or not) TOS episodes as they pleased - modern Trek was not ever going back there (Roddenberry was initially against ANY connections to TOS)
Then in season 6 Scotty turned up and the Holodeck depicted the TOS Enterprise in full 1967 awesomeness, with ZERO updating of the design. On the one hand this was a lovely bit of fanservice, but conversely this locked the production values of a decades old show down in-universe. From this moment on Trek was committed to maintaining rigid continuity with its past incarnation, whether it made sense or not. The more generalised "interpretation by era" proposed by the TMP novelisation was gone forever.
And I'm still in 2 minds if that was a good idea or not.
