You demand a "clear" answer but the problem is: in this case it's not just a simple yes or no.
Yes, you can use other skins **on your own risk **. For example if you use a skin/a skin-programm that violates Riots stance on certain aspects you can expect a punishment. For example allowing you to use in-game purchaseable skins for free is a big no go.
Besides that: I took a look at your link and have to say that I wouldn't recommend downloading it. The presentation is done poorly and seems like "Trust me, I say that and so it must be true" - like you already mentioned: some employees say no and other yes while the creator (?) here states for 100% that you can't get banned. His only proof is his word and a screenshot of a seemingly old comment without a direct link to the full discussion. Plus a few users in the chat said they got banned. And most importantly: the video shows that you can use skins that should be only available if you buy them directly from Riot (Nightbringer Yasuo) which will lead to a ban as ...
We reserve the right to take action against applications that allow the use of our assets for free (often referred to as skin hacks).
Here you find a bit more information about the usage of Third-Party-Software:
https://support-leagueoflegends.riotgames.com/hc/en-us/articles/225266848