If it's just self-hosting that you're after, then yes - open-sourcing the server-side components would definitely help. Regarding their marketing as "open source" - I haven't seen their marketing claims, so I'll take your word for it, and in that case I agree with you - it's a disingenuous claim.

But my comments were mostly around your original post, where you said you were sceptical of their trustworthiness because they hadn't open-sourced the server-side components. This implied that you would trust them (and I assumed the SaaS service) more if they did this. I was just expanding on a theme that you mentioned: "unless they willingly serve binaries that are not actually built from that code...", and I was just highlighting the fact we would never know if this was happening. So if you're sceptical without them open-sourcing, you should remain sceptical even if they did open source the backend.