Yes but in this case, they (TC) know that Arrington is a partner at SoftTech VC, and they also know that SoftTech VC is investing in this particular startup being covered (They mention them specifically in the article) so the ignorance argument is invalid.
Since someone within TC stands to profit from the company they are writing about, isn't a disclosure a good idea? I agree that it's not entirely clear-cut what to do here, but IMO to build any sort of journalistic integrity a media outlet should err on the side of disclosure.