Oh, that just happened. We didn't have established processes for promotions for a very long time. The company was a tiny startup when I joined (quite literally in the cellar of the company founder's place), with a really flat hierarchy and no distinction in seniority.
At the point when the company started to set up a formal process for promotions, I had already been there for so long, that I was considered one of the most experienced people, and that's how I ended up being filed under "senior coders" in the employee list basically since that category existed... It also was a bit weird, as that happened to coincide with all the COVID lockdown chaos, and I never had a formal promotion talk, just an email with an amandment to my contract, which I didn't even read too carefully, so I didn't realize at first that this was not just the yearly pay increase 😉.
Oh, and believe me, the impostor syndrome is strong with me. I would not have promoted me to that role.
I'm in a megafirm you've probably heard of, but I've been in this role for my third and fourth YoE, so I'm wondering how long it should take me to get to mid-level/senior.