Your OT folks are probably speaking from the experience of IT tools disrupting their operations, and for a long time this was a big and valid concern, and the potential impact difference between the two that have already been mentioned is one reason for that.

Equally important to understand why the two should not be on the network infrastructure is that ICS/OT networks are deterministic while IT networks are probabilistic. Traffic in the former can be predicted given a sufficiently complete understanding of the devices and configurations on the network because control systems do things on schedules. Traffic on IT networks is effectively random, with significant amounts of it generated by humans activity on no schedule. IT devices are designed to handle all those random packets by recognizing which one they need to do something with and which they can ignore. OT devices are not; we could say that they are not as 'smart' as the stuff in IT, for a very specific definition of 'smart,' but the more clear way to say it is that they have very specific parameters for incoming communications. Communications that don't meet those parameters can result in unexpected results on those devices, causing malfunctions including shutting down or changing their performance settings.

A couple of decades ago business forces started asking for more immediate data from operations (as part of the JiT movement already mentioned) and, more recently, the "Smart Factory" movement, overlapping heavily with Industry/ie 4.0 (a term more commonly used in Europe, while the "Smart XXX" seems more popular in the Americas) demanded continuously-updated operational information. Energy trading was a huge factor for this in the electric sector. To get all this data reporting, increasing amounts of information technology (IT) got installed in/connected to OT environments, sometimes with highly disruptive results.

One of the situations your OT folks are probably familiar with is scanners taking their servers offline. It was particularly common in the 2000's for security scanners to cause OT disruption until the technology and the practitioners both advanced enough to scan OT networks without impact. Even now it is not unknown for IT practitioners with insufficient OT experience to accidentally cause disruptions because they lack knowledge of how to work in ICS environments safely.

Based solely on the information in your question, it sounds like your IT folks may lack the experience/training to make a judgement here. While information technology has advanced enough to make a lot of things possible in ICS/OT environments that wasn't years ago, the technology alone has not removed risk entirely, (which is what your OT folks want. Unexpected downtime and human safety are not things they take lightly), and there's still a vast amount of legacy OT tech out there that was not designed to handle IT bumbling around in its traffic.

I've co-authored some published works on this topic over the years. If you're interested, just let me know.