I didn't say that Conversations from the Play Store is significantly more trustworthy in this regard than WhatsApp from the Play Store. I said that an app - such as Conversations - that you can build from source or download from F-Droid is more trustworthy than the Play Store version.

WhatsApp is a proprietary app and as such it's only available on the Play Store. Conversations is open source so you can download it from the Play Store, or from F-Droid, or compile it from source. So if you care, you can be significantly more sure that your version of Conversations "does what it says" than you can be of WhatsApp.