You absolutely can use MileagePlus miles to redeem flights on Turkish, assuming they have availability. What articles like that don't explain is that airlines are often stingy when it comes to releasing seats for award travel, especially to partners. Turkish is actually fairly liberal with theirs, but it's still entirely possible to want to use your miles and find that you can't, especially if you don't have a ton of flexibility regarding routes and dates.

Still, if you do a search from United.com for award flights, you'll find flights on Turkish (along with other United partner airlines) whenever seats are available.