Imsims are about immersion, sure skyrim has interactivity but it's world isn't believable due to ludo-narrative discrepancies caused by invoking too much of the TES universe into the game, the devs didn't have the money and manpower to build it's world WITH it's game play.
A Skyrim which is heavily modded with new game mechanics can be an immersive sim.
For example skyrim is a nation with 5 cites that occupy a fraction of a territory of 36km2 it's "urban" and national population correspondingly are ridiculously small.
The city of versailles is nearly as big as the nation of skyrim with it's 26km2, that's not normal what's even less normal is the fact that Tamriel is equal in size to 6 or 7 of Yvelines department 259 communes.
Yveline department is 1 out of 104 that compose france.
Skyrim vanilla is not imsim because
1. it's scope is too large and it's content is too small.
2. the game isn't finely tuned enough to have ludo-narrative coherence.