I suspect that it had to do with the WGA contract at the time that TNG was in production.

Writers with a script credit got specific residuals for characters they created in a script according to what Beyer wrote. So, taking a one-off character and putting them in a main cast role in a new show would be providing creator credits for the entire show.

Chief Miles O’Brien is a different story altogether. Colm Meany was originally contracted as a background regular as the transporter chief but without a name. These kind of roles are reliable and keep actors eating while allowing them to audition for other roles.

According to what I heard at a Creation Con panel back in the early 90s, the showrunners eventually decided to give the character a name. Colm Meany wasn’t advised, nor was his agent — so Meany actually was raising a complaint that he was being replaced without notice, only to find out he’d been upgraded to a named part.

It’s not a fan novel.

The Full Circle Voyager series is official tie-in fiction published by Pocketbooks and CBS. Official tie-in fiction is beta-canon. That is, canon until overwritten by something onscreen.

The key thing in this is that the tie-in writer for this series Kirsten Beyer has significant credibility and authority within the franchise. Her research and statements on the legal situation around this are credible and official.

Kirsten Beyer was in the middle of writing the Voyager books when she joined the television writers as of the first season of Discovery. She’s officially got the mandate as the expert keeper of canon in the writers room — even if she’s not always listened too. More, she was made the liaison for the tie-in writers to the shows as of 2017. She’s now a CoExecutive Producer and cocreator of SNW.

When that book was published, and she chased down the history on Locarno/Paris, she had the greatest access to past Executive Producers and internal legal advice and archives. She wouldn’t have claimed that without solid evidence.