These results look like sajaegi, Babymonster having a huge fandom and popularity, sold the same amount against a group that is only played in shopping centers. So I would not believe in the reliability of these indicators.
In general, Illit's success is so inorganic that I don't believe at all that they have popularity comparable to their positions in the charts. Since their debut, this group has been involved in Spotify boosts, and now they are simply taking advantage of the results.
You are confusing the timeline.
Babymonster debuted in November 2023 to a digital single that was not well received and missing a popular member. It was considered a low effort BP ripoff that was years out of style.
ILLIT debuted with a huge bop that was very well liked, so its debut sales were very good.
Babymonster first album and single Sheesh were awesome and well liked. But many fans in Korea took a wait and see approach, but they still had very good sales of 400,000 albums the first week. Remember that BM had a huge SEA fanbase, but those fans don't buy albums.
NOW the Babymonster fanbase is growing in SK and getting a lot stronger, as they realize this is a very talented and appealing group with staying power. They are killing it in the live shows and variety. ILLIT is stuck in a turf war with New Jeans and they are likely going to get hurt from further.
Babymonster has been destroying ILLIT on fan votes for the music shows, but ILLIT's Magnetic is a killer bop and just killing it on streaming. It is a very much a viral and mainstream, GP friendly hit. There is no denying it. But it isn't easy to replicate them and Lucky Girl Syndrome, a pretty good song, isn't charting at all.
The next album for Babymonster is likely to have a much higher first week sales than ILLIT. If ILLIT can still produce bops, they can build a good fanbase, but it seems just much more casual fans than Babymonster. But ILLIT just doesn't have the talent to probably keep on producing hits over and over. They are cute as hell and will play well in the Japanese hand shake market, but if they produce duds like STAYC does once in a while, they will fade in popularity like GFriend or Itzy did after coming out with a bunch of meh albums after their first few huge hits.
Anyways, that history makes that ILLIT had better debut sales plausible because of the timing and failure of Batter Up. But that advantage is pretty much gone with the knife that MHJ stuck in ILLIT and BM's talent and appeal.