Weird that the article doesn't mention this at all, as it's pretty relevant and certainly puts the 1:22 video on the Standard's site (with multiple, obvious cuts at odd points) in to perspective. This was the most-upvoted comment when this incident was last discussed, incidentally.
It has already been investigated internally (hence 'management action', which could mean a lot of things, including-but-not-limited-to advice to be aware that things might be taken out of context) despite the chap not making a complaint (up to you if you believe his rationale for not making a complaint, it does surprise me that he thinks he's the 'only person' that will get justice if he legitimately complains about a racist member of staff though).
The IPCC are picking this up as it was a national news story so need to ensure full transparency (hence "to maintain public confidence in policing"). It's certainly the right thing to do, but it's also worth bearing in mind that this does not mean that any wrongdoing will be found.
(For the avoidance of all doubt, if the original video has only been edited for stylistic purposes and/or the general context of their discussion holds true, then this officer is completely out of order and should absolutely be held to account for his actions. Equally, if the Met's internal investigation turns out to be lacking then I would expect action to be taken there too.)