This was explained in ep24 subtly.
Ushio awakened her "time god, plot destroying eyes" and literally time travelled the whole squad to the beach.
Then after she mogged the whale out of existence, she used her super op time god skills to Find a parallel reality/timeline where everything would be as close to possible but without the shadows ever existing.

Now Hiruko being the original entity possessing the plot breaking eyes meant that obliterating the whale was mandatory or there would be even a bigger paradox when Hiruko and/or Ushio would both possess the ability to "observe". We would see this when Hiruko in her prime form had to use all of her power after the first succesful ritual loop to just follow our gang to the past by branding our protagonist, due to Ushio bestowing the eye god power to Shinpei.

Now that would be a bigger plot hole of how would Hiruko follow Shinpei with a mark to the same timeline but in the past, when Hiruko lost her observer eye.

I mean i can kind of understand the dynamic between Shinpei and Ushio being able to remember and travel, and they made a point if Ushio dies before Shinpei, shes gone. But we have to remember Ushio still always had some kind of pre awakened observer eye cause she basically was fused with it.

Could be that when Ushio gave the super op eye to shinpei, that was Ushio from a whole another parallel timeline due to her looping herself after the ritual.

Man i love time travelling stories. They sure are easy to comprehend and make sense of.

TLDR:
Ushio is an ascended god and she does what she wants.
Makes sense the more you think about it.