I would take the SeatGuru comments with a grain of salt. I find that bags that are within AC's permitted dimensions fit fine.

The comments probably arise from two factors. The first is that there are plenty of people whose bags are actually outside AC's legal dimensions.

The second is that there isn't enough room for everyone to bring on a roll-aboard that fits those maximum dimensions, but a lot of people do. This leads to people who board late on full or fairly full flights frequently needing to check their bags, regardless of their size.

Your bag sounds like it should fit okay, but if you have zone 4 or 5 boarding, you might get stuck checking it if the plane is full.