Bit of a panic with my car tonight... Possible causes?

I have a feeling used cars are going to be a while. Even after the chip supply chain is fixed and new cars are plentiful, the dealers won’t be able to (significantly) bring used car prices down to a normal level until they clear all the expensive ones off their lot and can replace them with more used cars that they didn’t pay an arm and a leg for.

I just traded in a 2018 Civic not that long ago. ~30,000 miles, 3 years old and I got something like $26000 for it. I have to imagine any one in the market for a Civic SI that’s willing to pay $26000 plus or minus a few thousand* will pony up the extra cash for a brand new one, if they can find one.

*the dealer may well take a small loss to get it off their lot, especially since me getting a new car from them was contingent on them buying this car from me.

Even if you have cable driven throttle control, you probably still have a throttle position sensor. There are three or more sensors that control fuel flow. Mass airflow sensor, MAF, Throttle position sensor, TPS, Exhaust oxygen sensors, O2. Even temperature and rpm sensors will effect the fuel flow.
A bad one of these in the system can cause more fuel than optimal to be input. This can increase speed to a point. But if you do not manually increase the throttle opening, the car will not runaway accelerate. The fuel amount will become too rich and actually slow you at some point.
It seems the computer is thinking there is more air/O2 in the mix than there really is, so you are getting extra fuel added to balance to best combustion. I would think MAF, TPS and O2 are the main suspects. There is also a idle air control, IAC. But that usually just messes up the very low ( idle ) speed of the engine. Like mine is currently doing.
If this has been going on a long time, your plugs may be fouled up due to too rich a fuel mix. Replace them with the actual fix.

Not with prices and availability being the problem it is right now. If you experience it again, and it proves not to be the mat, take it back to the mechanic. A.) unintended acceleration is not something you want to chance, and B.) quick fixes to stop a leak can get really expensive.
Also C.) if you haven’t done a replacement on most/all rubber parts that can be reached, its past time to get it done.