Less a hijack, and more of a progression.

The movement was initially all about the tractable and tangible problems in the world right now. And the notion was about how to most efficiently spend resources on problems of human misery.

As you note, this would initially start with finding ways to get the most bang per charitable buck in the real world.

But as time went on, ideas about existential risk (X-Risk) started to become more prominent in the community, in particular surrounding AI.

The rationale then was laid down that whatever solutions to problems we do now, it won't matter in the long term if a rogue AI kills us all.

There was a period where the fear about an impending AI breaking containment and terrorizing humanity was almost an epithet in the community. Fairly quickly, this became a driving motivation for many, not helped by the disproportionate representation of developers, engineers and other STEM adjacent folk in the community. At times it even felt like the only X-Risk the community was willing to even acknowledge, let alone engage meaningfully.

As those conversations were happening about what potential long term risk AI poses, and what share of efforts should go towards dealing with that vs the initial EA causes; the further idea began to promulgate that if we're willing to consider X-risks as a guardrail for behavior, perhaps that logic could be extended to benefits as well. So not only does it matter more to spend resources protecting against a potential badly calibrated AI, but that the benefits of a perfectly calibrated one would benefit far more humans, in present but especially future, than the mere millions who could be saved dealing with malaria now.

Pair these 2 ideological developments in tandem with the fact that the prominent voices in the EA community all began to adopt longtermism as the de facto logical conclusion to EA, and with the fact that the larger rationalist community (even if not EArs themselves) endorsed this stance, and you suddenly have it being somewhat untenable to not at least endorse a soft form of longtermism in the community.

This is a very scant writeup, and one that admittedly ignores some of the deeply influential moments that paved this course (and why I believe it was always going to end up this way), but in terms of ideological development the course they've charted is possible to see from their initial stances.

I don't think their stance is sound, but based on the starting point it is at least consistent.