“Working class” refers to people whose main source of income is a salary earned on exchange for labour, in a free contract between them and an employer.

“Capitalist class” refers to the ones purchasing the labour power of others in order to have them work capital that the former own.

You’re confusing “working class” with “lower class”. The concepts of low, middle and upper class are quite diffuse, mostly based on income, and used to draw artificial barriers between workers who at the end of the day have more common interests than different ones.