This is more of a hot take friends, as these terms often get used interchangeably. Rather misunderstood as well!
I think a lot of it originates from the ecommerce world, and so things are very technical or sort of backend mentality. Logic based, rather than frontend, personable based.
As someone who is more of the latter I have found that these terms mean something else when used this way. The premise behind "Multi" is to use "more than one channel", and the premise behind "Omni" is to use "multiple channels cohesively". As in making a seamless experience for the user.
Where multichannel is a bit of a simplified version of omnichannel, but the latter is more about the user rather than the creator.
I think a lot of people unknowingly use this approach, where they syndicate or spread across multiple platforms. Not only is that a one way ticket to burnout, as you are spreading yourself too thin (funny that this is coming from the polymath guy!). Additionally, you are treating each platform as individual channels.
Real World Definition
Getting the base level of convenience, which is like saying "Oh I made account on this platform". Not actually using it, but if a follower wanted to find you. At least you have an account to follow, but with no content.
It is that superficial level of effort.
Every definition I see out there talks about it from the viewpoint of ecommerce, but that isn't the only business modality that we should account for. Not only that it isn't the only use of the word.
Real World Definition
When you take an omnichannel approach. omni meaning every/ALL, then you focus on all fronts. It may seem overwhelming, so for those starting out perhaps don't go that path yet. However I think even really early on, that you should be sowing the seeds in your own strategies to reach for omnichannel.
Rather than "business strategy" we are thinking of "content strategy", and while "seamless and effortless" still checks out. The mindset of a sales vs marketing team member will prove the difference between approaches.
Why you should be OMNI not MULTI
Perhaps I'm thinking too much into it, but I made my previously named series "OmniContent" based around that concept. I only changed because ironically an omnichannel business service had a "OmniContent" as one of their services, and I didn't want to catch a case of the legal action.
PolyInContent is the name now, which is why this series is called the PolyInContent Digest.
The series (different from digest) itself is relatively simple to make, as it starts off as a blog post or video, turned into the other, and then cut the audio. From there you chop it up and repurpose.
The point is that this series will have the same feel across platforms, and despite it being one episode. Each channel gets a similar experience, and the omnichannel feel across the board.
When a user finds one, then goes to another, they will feel the same way (ideally).