In other words, in that application, the ‘spouse’ line makes sense – the spouse is not the principal applicant, but is _an_ applicant.
For family sponsorship, there is the sponsor and the principal applicant (and any dependents such as kids of the principal applicant). Key difference is: there is no spouse-applicant.
Note you actually have to be a bit more careful as some forms will ask the PA to identify their spouse, but note, NOT as an applicant, just for identification and background purposes. Likewise for example children who already have Canadian citizenship cannot be applicants and should not be on forms listing applicants, but will be on forms asking for background information.
Unfortuantely I don’t think I can explain better than this (helped me understand, ignore if it doesn’t help you), but once you get the idea that some of the forms will refer to a ‘spouse’, and in that context they mean the spouse of the applicant (assuming basically that the spouse is the dependent here). Spousal sponsorship means the principal applicant doesn’t have another spouse who’s applying to become a PR, obviously, since the spouse is the sponsor.
So for spousal sponsorship, the key terms are sponsor (spouse of the principal applicant); principal applicant (spouse of the sponsor); dependent children (applicants – ignore Canadian citizen children for the most part). There are some more detailed situations – e.g. children who have children, accompanying/non-accompanying dependents, but to start the three above are the main terms.