I cannot speak for Jane Street, in particular, but I have spent many years working at a very similar firm, and Investment is not Technology.
There are engineers working in Investment, yes, but the risk profiles, the payout profiles and the focus of their work is very different. Investment deals with getting and analysing data, understanding the market and their algos, coming up with trading ideas, handling their PB relationships, doing the actual trading (e.g. portfolio optim, factor hedging, worrying about financing and borrow costs) etc. Technology deals with all things infra and (in most places) ingesting data and making it easily available internally.
In particular, I would be surprised if the interns in this article were on the Investment side - a fund would rarley publish anything remotely resembling research. In fact:
"Yulan ended up working with both the trading desk that was running this process, as well as the research group, which pointed her at some cleaner cost functions to optimize, as well as encouraging her to try out simulated annealing in addition to the greedy algorithm she started with."
The "research group" and the "trading desk" are Investment.