Yes, I would like to know where you get that from, too? Of course Renoise is not bad here, and it comes with a good compressor set and a great maximizer, too. Then you can use the usual suspects of mastering plugins. I only think Renoise can be annoying regarding some aspects, e.g. fine editing vst automation or even per note expression for vst3 (not possible). Ever tried to fine edit a virtual guitar plugin so it sounds really real using Renoise? Such tasks will be way more easy in another DAW. If you stick with Renoise sample instruments, you will get most of these features - if you love pattern commands. But the VST part is put quite a bit on second place.
I think there are some parts in Renoise - depending on your personal workflow and goals - which could under circumstances be a handicap. If you like it, then use it.
There is no technical reason why Renoise should be worse in mixing / mastering than any other DAW. I would even say there are some productions out there, which can convince you of the opposite opinion.
Then again, some mastering legends seem to heavily automate the stems while mastering. Such automation job then is more easy and precise in a usual DAW.