Asako's confession seems sincere, but it's oddly brief (taking up only 7 pages of 29). Usually the character in this situation narrates their intent to confess in the previous chapter, and then during the confession, elaborates a little, e.g. on how they figured it out. I mean, Asako even says "the answer was right in front of me", but "I want to be with you" isn't a very detailed answer.
There is buildup to Asako's confession through the previous chapters, but very little in this chapter itself. There is some more context in her conversation with the sis-in-law, but we already knew that Asako liked to take care of her partners. The way Miyako breaks the pattern is that she is receptive to Asako's caretaking, and was the one to pursue Asako first. Yet the idea that Miyako is not used to being appreciated and treated with warmth by her partner isn't emphasized or reiterated here at all.
That said, I am of course happy for the two of them. I just hope that this rushed feeling isn't the sign of a premature end of the series.