The affix kan comes after the word bṛhati when it means 'an article of dress'.,
The word कन् is to be read into this sutra and not its prohibition. The force of this कन् is nil, or it is an example of स्वार्थे कन् ॥ Thus बृहती + कन् = बृहतिका (प्रावारः) 7.4.13 (by which long vowel is shortened) \an upper garment or mantle\.
Why do we say \when it means dress\? Observe बृहती छन्दः \the metre called Brihati\.,