I was thinking about that the other day. It must have been a hard sell to get that 90 minute episode in. It makes sense, to anchor the characters to their mother and provide a driving motivation to them, without giving the viewers a bait and switch.
The justification here is probably just that "wow, look how popular Oshi no Ko was, let's do what they did".
Freiren: Beyond Journey's End anime to debut with 2 hour long episode (kbin.social)
https://www.animenewsnetwork.com/news/2023-06-29/frieren-beyond-journey-end-anime-2nd-video-unveils-2-hour-september-29-debut/.199742...