So you're saying the only available production master would be the vinyl master and so they used that because it was cheaper and more convenient than creating a new production master for CD?
Fair enough, I can see how that could happen.
Not quite.
Briefly:
Traditionally tracks were recorded and edited and mixed to tape(s*) to the artists & producers' satisfaction , by recording & mixing engineer(s).
Then that original mixdown master tape was further 'mastered' for LP, because the LP format (and playback technology at home) had certain physical constraints (e.g. end of side had different constraints than start) and because 'mastering' also involved such aesthetic mandates as making the tracks 'hang together' sonically as a group -- by adjusting level, stereo width, tweaking dynamics and more...and such trivia as putting the right-sized spaces between tracks. This was the purview of the 'transfer engineer', who evolved into the 'mastering engineer' (and to an extent, the cutting engineer). The result was a production master.
This practice was carried over to the digital era ....sources were 'mastered' for CD release, involving the creation of a digital master as the 'production master' for the CD*. Atlantic Records, for example, early on employed at least two CD mastering engineers (Barry Diament and Zal Schreiber) who were often not credited, but were responsible for mastering many of the 1st generation CD releases of, e.g., Led Zeppelin, AC/DC, Yes, CSN&Y...
What I (and they) say is that the tape source THEY WERE PROVIDED WITH to create their CD master, was often (not always) an LP production tape, rather than the original (mixdown) master tape(s).
*e.g. plural as in splicing together tapes of finished tracks to create one tape per 'side'
(**it was/is of course possible to simply digitize an analog tape 'as is' (a 'flat transfer') to serve as the CD master. 'Mastering' then entails a more limited set of choices.)