While I absolutely visit the site, I don't really regard nbadraft.net as "well trusted" - sometimes I think they've got a screw loose.
BUT
Regardless of what they may have done in terms of updating their 2018 mock draft post-hoc to match the real draft, it does appear that they did have Bagley in the #2 slot for much of the 2017-2018 mock draft year.
For instance, here's a internet archive hit on Bagley's page from June 2 2018 (so 3-4 weeks before the draft) that shows he was mocked at #2
https://web.archive.org/web/20180602082905/http://www.nbadraft.net/players/marvin-bagley
To reassure you that somehow this page that was archived before the draft didn't somehow get updated later, here's an internet archive hit from December 4 2017 showing Bagley in the #1 spot on the mock:
https://web.archive.org/web/20171204213356/http://www.nbadraft.net/players/marvin-bagley
I looked for archived versions of the mock itself, but there don't seem to be any of the 2018 draft from the time of the draft - my guess is that they changed the URL of the mock draft pages sometime after 2018 and that if one knew what the URL was back in 2018, one could find archives of the full mock. Nonetheless, for looking at where they had Bagley mocked, the Bagley page will suffice. And I didn't look through every hit (there are a bunch), but it looks like they basically had him mocked at #1/#2 all year.