Ignore that CSS file as it's the fallback CSS file and not the one used in generating the 'all' file.
The CSS actually comes from a mixin:
@mixin text-truncate() {
overflow: hidden;
text-overflow: ellipsis;
white-space: nowrap;
}in /theme/boost/scss/bootstrap/mixins/_text_truncate.scss and used specifically in /theme/boost/scss/bootstrap/utilities/_text.scss, see https://getbootstrap.com/docs/4.0/utilities/text/#text-wrapping-and-overflow.