There are edge case models where this is not true at all.
For starters, for all the non-Reforged models that are "Classic" graphics, the Reforged game changed the format of every single file a total of 4 times. So if you think about something like the Footman Classic model, this has gone through 4 iterations of format changes from 2019 until 2025 when the most recent change was made.
As far as I can tell, most people retroport the model to the 2003 Frozen Throne game format before uploading to Hive, since those retroported models still load fine in the present day Reforged client next to its newer format files.
However if I do a survey of recently uploaded model files, I can see that a model such as
https://www.hiveworkshop.com/threads/united-alliance-sentinel-tank.370970/#post-3713152 by the user
@General Frank will not load on patches older than Patch 1.29 and we can detect this by looking at data and information in the model, without bothering to test the model in-game. But nobody has categorized which models depend on Reforged-only textures, so there is not a suggest patch version or warning number for you when downloading a model (unlike the map section which tells you a target version for maps).
Frank's model won't load on Patch 1.28 and below because it uses a texture file from inside the game database that those game versions do not have (
ReplaceableTextures\TeamColor\TeamColor20.blp ). However, you could just as soon have a model uploaded directly from the Reforged game that was in the 2022 format, and it would render fine on Hive and look like a Classic model but it would not load on game versions prior to 1.33. Or you could have a model in the 2020 format that would not load on game versions prior to 1.32. (But I went looking for an example of a model like that in the recently uploaded Classic models on Hive and did not find it -- most people uploading files to Hive are kind about using the retroported 2003 format.)
So it's certainly possible in multiple ways for a model to "sneak" into the Classic section that doesn't work on a truly classic CD game version of the game.