No, there is no IEC terminology for a delta system at all, ALL power systems are Y of some sort everywhere except North America. That's why most Asian drive mfrs ignore this issue in their technical data; we represent a tiny fraction of their world wide sales. T-N means a Wye power system, so they are telling you that it is ONLY designed to be connected in a Wye system, but "contact us" if that's not what you have. But then nowhere in their available tech support documents does it actually say what to do, even their phone tech support people are untrained in this issue. I used to be a Yaskawa official integrator via Magnetek when they were the main "brand label" for Yaskawa. But after Yaskawa bought out the Magnetek contract and fired all of the rest of their US partners, they lost a lot of the "tribal knowledge" of the established sales and support teams these companies had put together. This issue frustrated the heck out of me and I complained about it a LOT, but it fell on deaf ears. They just didn't get it. Back in those days I WAS eventually able to find a single US based Engineer on their staff in L.A. who understood this issue and his solution was: install a Delta-Wye drive isolation Transformer ahead of each drive (or group of drives). While this is a solution that I usually recommend because of what you expose the VFD to by removing the ground reference, it is sad that once that one guy at Yaskawa left, nobody there picked up the ball.
Bottom line, ALL VFDs that have been designed for Y systems, (meaning all VFDs really) will have an issue with a delta system, corner grounded or not. I suspect that your Yaskawa representative was unaware of this issue and gave you an answer that would make you go away. One of their tech support people once claimed that the MOVs on the diode bridge (the main issue with a Delta system) are connected Line-to-Line, so it's not a problem. But that CANNOT be true, because all CE labeled equipment, which includes everything Yaskawa makes, is REQUIRED to be referenced to ground, that that is inclusive of the MOVs on the front end of the drive. In fact on the VFDs that are available where you are given the option of removing that ground reference (A-B and ABB for example) to use them on a Delta system, they will warn you that doing so violates the CE and UL listing of the units. So the fact that a Yaskawa representative made that statement proves my theory that there is nobody left there that truly understands this issue.