Adding encryption you control inside an iMessage transmission can provide more assurances that your messages remain unreadable to others, but there a whole lot of provisos you need to consider before accepting this as a higher level of security.
It’s nice to see reviewers of applications present the concerns, first, before what might be nice about new ‘security’ apps. Namely that crypto is hard to do, not all crypto is the same, and there are basic questions concerning the reliability of the companies providing the security assurance.
More broadly, that applications can route double-encrypted messages through Apple Messages will not necessarily enhance security but, instead, mean that comunications are only as secure as the application applying the second layer of security. Apple is a great big target that everyone wants to penetrate and so Apple hires terrific technical and legal staff to keep government and others at bay. Can we expect that app developers selling encryption apps for a dollar or two will possess an equivalent commitment and competency?