If it is called sexist for that reason, then the person using the term 'sexist' to describe the situation is comically misunderstanding the term. Sexism is basing one's beliefs about another on the basis of their sex, even if those beliefs don't actually relate to sex in any meaningful way. For instance, saying "The IT industry requires you to work hard, so women won't like it, because y'know, women can't stomach hard work" would be incredibly sexist. Conversely, saying anyone is free to join the IT industry is the exact opposite of sexist.
Unfortunately, there seems to be a bit of a misunderstanding around what sexism actually is: it's not sexist for something to simply not be favourable to some sample of a given sex. In order for something to be sexist it either has to make assumptions about the qualities of a given sex based only upon their gender, or be discriminatory toward a given sex based solely upon their gender. Discriminating on the basis of hours worked is the opposite of both of these.
Unfortunately, there seems to be a bit of a misunderstanding around what sexism actually is: it's not sexist for something to simply not be favourable to some sample of a given sex. In order for something to be sexist it either has to make assumptions about the qualities of a given sex based only upon their gender, or be discriminatory toward a given sex based solely upon their gender. Discriminating on the basis of hours worked is the opposite of both of these.