that only includes real MS-DOS games
No. There is no strict tag enforcement.
Games in https://itch.io/games/tag-ms-dos and https://itch.io/games/tag-dos do not promise that they run under a specific version of any DOS.
A tag only means that the game has something to do with the tag. So even games that just look like a DOS game might use it.
The operating system meta tags do not even include the OS version. It just says Windows. Which Windows? Windows 3.11?
In theory, you could try to establish a tag that means, that a game has a version that will run under a specific 30 year old DOS version. But as no time pointed out, the challenge is, to convinve other people to use it.
If the download file is big, maybe there is freedos included. Or, it's just a game simulating the dos style, but the description should tell you more.
And if you missed it, tags are not created by Itch. The list of tags you see, is just the "suggested" tags, which basically is self referencing, since it just means, the tags are in that list. There is https://itch.io/games/tag-386 and https://itch.io/games/tag-486 , but only a handful of games use it.