It’s not technically matte in the way that old time TFT matte displays are, but it’s not anything like glossy, either. It shows virtually no reflections under bright lights or in a very sunny room.
I’d also like to point out that that 27UD58 model regularly goes on sale for around $300, occasionally under; but the bezels, enclosure and stand are awful; but is a very good panel; identical to the panels in most other affordable 4K displays, as most are made by LG anyway; so if you don’t care about the enclosure, and are willing to pop $30 or more on a good VESA mount, it’s a great, affordable choice.
The panels used in that one are similar, but there are better specs available on the 27UD68 and 69 series, and those are regularly on sale for $400 or less; I even saw the very best version, the 27UD68P-B (best NIT and color), go on sale for ~$370 on Black Friday.
Best Buy usually has the 27UD69-W (white enclosure) for around the $400 regularly; it’s a fantastic screen, with the nearly-matte finish and super-narrow bezels, great for multiple displays.
I have two 68P-B, and one 69-W, side by side, and I swear I can’t tell the difference, nor can others who’ve tried without looking at the color of the enclosures on the back, but I also don’t need them at full brightness (I run at around 70%).