The marriage wasn’t sacramental because sacramental marriage requires both parties to be Christian, this has no bearing on it being a valid marriage.
Two non Christians have a valid natural marriage
A marriage of one Christian and one non Christian is a valid natural marriage.
If two non Christians later become baptized their marriage becomes sacramental.
A marriage was either valid or invalid at it’s inception, nothing that happened after the fact changes that. A marriage tribunal determines if the marriage was valid at it’s inception or not.
All marriages are presumed to be valid unless proven otherwise. There are a variety of reasons why this may be.
Certain rules apply to Catholics that don’t apply to non Catholics. For example, Catholics are required to marry in the Church by a priest, unless they are granted a dispensation.