What are they? Furniture increases your stats. Showcase items do this. Pets do this... so what exactly IS a stat item?
Just ignore any nonce who says "stat item". It's so cringey. Most of them don't even know what it means but say it anyway. For some it means showcase items. For others it means story rewards. For others it's anything that increases stats... until you point out that furniture and chibis do that, and then they twiddle their thumbs. Really, just ask them to clarify and they probably will. It saves them from humiliation and keeps the tone light which helps if you want to actually get trades.