Asexual means a person may deeply fall in love with someone, but they don't experience sexual attraction.
Aromantic means a person may experience sexual attraction to someone, but they don't experience romantic attraction (the kind of attraction that makes someone want to date or be in a romantic relationship.)
A person can also be both "aroace", meaning they experience neither sexual nor romantic attraction.
In this manga, the MC is "asexual", but not "aromantic".

I remember reading somewhere that japan doesn’t have a nuanced term for aromanticism and it’s generalized as asexuality. (please tell me if i’m wrong)
this is a great story! I just hope if it ever gets an official english translation, they localize the terminology as well.
my interpretation is that neither of them are aromantic and their relationship is actually romantic, but ML actually is asexual. I love stories that explore these identities!!