I agree overall - there are three different points here (IMO):
1) It is possible it's not a spousal sponsorship - in which case the actual move would be to remove the partner as a dependent spouse from the app.
2) On the more innocuous side, 'application' is a bit ambiguous - in reality it's basically a joint application, but in some contexts, may be considered two - application to sponsor and application to become PR. Plus the term 'applicant.' It's probably true that the sponsor can't withdraw the /applicant's/ application [to become a PR].
But if one is thinking of it this way, it should be perfectly obvious that the application to sponsor CAN (and hsould) be withdrawn /by the sponsor./
[That will effectively kill the sponsorship / spousal application of course - if in fact it was a spousal sponsorship.]
3) You bring up a very good point, that everyone should always keep in mind that lawyers work for the person that hired them. That does NOT mean the lawyer can lie to another party (I think - not a lawyer myself). But they may not always formulate things in the way that's in the best interests of the other party.
The more probable answer may be rank incompetence - but either way, the OP should be cautious about this advice.
And yes - withdraw the applicaton to sponsor ASAP.