Yes. In addition to that, unless I am badly mistaken, mobile SAMs don't serve the same role as other siege weapons. Other siege units exist to bombard large stacks, reduce the defenses of a city, or both. Mobile SAMs aren't something you use to soften up a stack or a city, they're something you park as part of your own stack to defend it against air attacks. They are never normally a good choice for attacking anything, they do no collateral damage, and they cannot bombard cities.
Now, despite this, it is reasonable to class them as a "siege weapon" in that they are large, ponderous*, require time to set up, and require dedicated security troops to protect them from enemy raiders. But while they are siege weapons in that sense, and while land-based cavalry units (including armored/mechanized cavalry) could reasonably be inflicting flanking damage to them... it does not make sense for flying units to be able to casually damage SAM installations. Because SAM batteries are specifically these terrifying deadly things for all aircraft, and even when aircraft do take a shot at SAM sites they generally do so as part of a carefully preplanned operation with all kinds of fancy diversions and distractions and aircraft whose only job is to jam the enemy radar so they don't put a missile up anyone's tailpipes.
They're not something you blow up on the way to your real target.
*A typical heavy SAM is the size of a telephone pole...
