Orochi was keeping the part of the agreement related to releasing the hostages, which means not everything that was promised was a lie. The only thing missing was the confirmation that once the ships were completed, they would actually leave Wano, something that could only be proven after several years. That is why the situation was fifty-fifty.
On one hand, part of the agreement was being fulfilled, which made it believable that they might honor the rest. On the other hand, there was uncertainty about a promise that could only be tested in the future. He wasn’t stupid, he just had no other options.