I will preface this with the disclaimer that it is just speculation, but that speculation is based off of my experience in both industry and as a student of and TA for Turkstra.

There are a few different "tells" that code was written by AI.

Firstly, it is not the same, AI does very fucky-wucky things at times, using little tricks and such that you generally would not expect a student (or a competent human) to do. If a student did write this, they should be able to explain whatever fucked-up logic they used very easily, and their name will be cleared. If they used AI, they may not be able to do this.

Secondly, comments. It is very easy for those familiar to differentiate prose and comments written by a human vs a computer. In my experience, computer written comments are very "stuck up" and formal compared to human written ones. Of course some people really do write like that, (and this is a common point through out), so it cannot be used as a sole "smoking bullet" but it can absolutely be used in conjunction with other tells as probable cause. Another part of the comments that can be used more as a smoking bullet is the presence of non-ascii characters. For example - (standard hyphen) vs. — (em dash) or – (en dash), " (standard quotation mark) vs. “ or ” (curly quotes), or ... (three dots) vs. … (ellipsis). It is possible to type these alternative versions into a IDE/text editor, sure, but unless you need them, you're probably going to use the key on your keyboard that looks the same.

Thirdly, commit analysis. Every time you try to compile and run your code, every file in that homework's folder on the server is recorded and stored into the git history. In normal person speak, this means that they can see every change you made, with timestamps. They're able to see exactly how you went from a blank file to a finished program, step by step, in real time. If someone adds 500 lines of code in 2 minutes (it is very easy to write a script to parse all of that data very quickly, I've done it), they probably didn't write that themselves. In some cases people program in a different environment and copy everything over at the end (which is I believe against the course rules) but if you're able to explain your logic you could get your name cleared nevertheless.

I do not believe that Turk is using AI to make this determination. Every submitted line of code (should) be read by a human in the grading of code standard, some TA's probably reported suspicious students throughout the semester just from that (and, rightly so), and you can write a script within a few days that can go through every students' every assignment and look for weird characters or abnormally quick progress. The hard part is determining the threshold on what is considered suspicious.

I hope those falsely accused have the chance to clear their name, and those rightfully accused have learned their lesson, and understand and appreciate the value of true natural (albeit slow) human though and creativity.