That's not how canon works. By that logic, Memory Alpha is canon, even though it's literally fanmade. Hell, the posts where I'm explaining various in-universe tidbits are canon with the exception of whatever conclusions I'm drawing from them. Using, reformulating and restating canon information doesn't make your product canon, just like how the Unofficial Holy Bible for Minecrafters is not an accepted part of the canon of the Roman Catholic Church, despite being a literal Bible. Only the Church could decree it to be canon, just like how it's the prerogative of ViacomCBS to decide what Star Trek's canon includes and what it doesn't.
If Trelane really did see 900 years into the past, he should've seen and reproduced things from the Hundred Year's War. You don't exactly need to be a history buff to know that the middle ages and the Napoleonic era didn't exactly look the same.
EDIT:
I'm a Star Trek fan. I would've watched the series either way, even though I prefer an update. I didn't have a problem with it looking the same in Trials and Tribble-ations and In the Mirror, Darkly. But the wider audience, including casual viewers, and new fans who came after 2009 or even started out with Discovery, would find retro visuals jarring unless retro is their thing. And of course they went for the TOS era when restarting the franchise after more than a decade of a break, Kelvinverse notwithstanding, and even that used the same reasoning. When most people hear the words 'Star Trek', they're thinking of Spock and his pointy ears, the Enterprise, and maybe Captain Kirk. Having a brand new series follow up after the TNG era with the occasional reference to Picard and Janeway just wouldn't have brought in the viewership numbers Discovery's TOS references did, that ultimately allowed the studio to begin branching out once more.