it is not possible to mistake the one for the other
You say, immediately before accusing me of lying about my positions, and assuming that whoever accused me of being a tankie must have been both correct and acting in good faith.
To be clear, I am not a strict pacifist, although I was at one time. The shift from pacifist to "tankie" is very small, basically just one exception. Before, I believed in "no war" and now I believe in "no war but class war." The vast majority of war and violence is not class war, it's just parts of the ruling class fighting each other over resources. There are only a handful of states that even call themselves socialist and none of them are at war, so that one exception isn't very relevant.
If I actually did "support" Assad, then how would that look different materially from opposing intervention? I'm an American, I have no control over Syrian domestic policy except through my state intervening. Unless I'm going to fly over there myself and pick up a gun on his behalf, or unless I'm going to somehow convince the US government to do a complete 180 and prop him up, then the distinction is merely rhetorical. And a rhetorical distinction, far from being "impossible" to ignore, is in fact very easy to ignore. If I were secretly an Assadist, then wouldn't I be advocating for the US to adopt the exact same policies that I would if I were a pacifist or isolationist? Then, if I advocate for those policies, it is trivially easy to accuse me of secretly "supporting" Assad, which is why people did so, and why they do the same in every other conflict.
This isn't rocket science, it's not some new thing. Accusing anyone who opposes war of supporting the enemy is a tactic as old as history. As Goering said, "All you have to do is tell them they are being attacked and denounce the pacifists for lack of patriotism and exposing the country to danger. It works the same way in any country." If you ever have the occasion to oppose a popular war, I can promise that you'll encounter the same accusations.