You're right, these three things are key to a true West Marches campaign:
1) There was no regular time: every session was scheduled by the players on the fly.
2) There was no regular party: each game had different players drawn from a pool of around 10-14 people.
3) There was no regular plot: The players decided where to go and what to do. It was a sandbox game in the sense that’s now used to describe video games like Grand Theft Auto, minus the missions. There was no mysterious old man sending them on quests. No overarching plot, just an overarching environment.
Ben Robbins, whose blog you quoted, should know, he popularised the concept!
There is a bit more to it though...
Implied, but not explicit in those first three points:
Every game session begins and ends at the same point of origin (normally a home town). This means:
- Every session can be entirely self contained (the new session will always represent a completely new foray, out from the base, into the wilderness, even if coincidentally the party is exactly the same as last time, and want to resume a similar pursuit.
- Players don't have to spontaneously appear in unnarratively satisfying ways when needed.
- In later adventures, as travel times increase away from that point of origin, the act of travelling itself is more likely to be handwaved / skipped over.
The players decide where to go and what to do in advance.
- Yes, it's a sandbox, but the players must decide what they would like to do in advance of the session. Normally, as part of booking the DM's time for the session the players would also state what they wanted to do in that session. This way the DM can prepare in advance, without needing to prepare the whole world, or improvise large peices of content.
Outside of those first three points:
Session reports are always shared
- As all PCs are assumed to spend their downtime in the same town, word gets round about what happens on each adventure. Players are encouraged to write up session reports and distribute them to each other. It's a living world, the same goblin warlord cannot be killed by two different groups, so the second group that wanted to try needs to know if the first succeeded or not.
- New quest hooks can be picked up by any group of players.
There is a shared world map, that's potentially unreliable
All initial objectives and later objectives that are discovered are marked on a shared map, which players can use to suggest placed they want to explore.
The initial map is produced in-game by a character and is only as reliable as that characters map-making abilities / trustworthiness. It is later edited by the players who may also make mistakes. This means it's possible to get lost, if the map is wrong (and the players can subsequently correct it).
Competition between players is actively encouraged
- Jealousy is considered to be a useful motivational force in getting sessions booked and games actually played. If anyone else can pick up from the interesting place where you last left off, or someone else has discovered something exciting, then that motivates you to prioritise organising your next session. Especially when magic items are on the line.
Content is loosely tiered
- Players started at a low level and would meet on average higher levels of danger the further they ventured outside of town. This meant players could largely assess whether a threat was likely to be appropriate or not.
- Significantly stronger threats in low level areas were normally well sign posted. Having pockets of more difficut enemies made the world more exciting, more diverse and incentivized PC's returning to earlier explored areas later, when they were stronger.
Matt Colville made a great video that covers all of this in detail, if you don't fancy reading all of Ben Robbins blogs.
However, when someone says they are running a "West Marches style" campaign they may often mean something much looser than this:
The only conclusive way to know what someone else really means when they say they're running a "West Marches style" campaign is to ask them.
That said, at a bare minimum it is likely that these things are implied:
They don't have a regular group of players, but pull from a larger pool each session.
Each session will be entirely self contained.
There's probably a focus on exploration.