I am not familiar with the ecosystem of geographic data and mapping as online services. Can someone please explain...
* How this tile format, or the organization behind it, related to OpenStreetMap (if it is related at all)?
* Why the need to replace the previous tile format / scheme which they mention?
* What challenges such a project faces (other than, I suppose, being noticed and considered for adoption)?
1) It's not. Maplibre is a JS library for displaying map data. OpenStreetMap is a collection of map data that is published in various formats. Different levels of the stack.
2) It's an optimization/advancement. There are some pain points in the older version that 10 years of experience can fix in a newer format.
3) Attention, funding. Technically, they're at the leading edge of open source.
The key info token you'll want to know as someone foreign to map topics is that maplibre is a licence continuity fork of the formerly open source Mapbox code.
Everything else pretty much derives from this, e.g. yeah, OSM did not suddenly go all in on former mapbox stuff only because the company started keeping updates behind a paywall, OSM continues to be as tool-agnostic as ever.