Just dropping in to say that the logical conclusion is just "they jumped the gun on announcing the engine".
Given what happened before, I'd say it's very likely any company doing a crowdfund would confirm with the company (and ensure both parties have that confirmation in writing) before proceeding with the crowdfund. Considering that all they did was delete the tweet that specifically said "YAO Synthesizer V AI", and they're now saying they cannot confirm what engine she's going to be on right now, it's pretty easy to connect the dots and say that they didn't have outright permission to say she's going to be on Synthesizer V, but that the project can proceed as is. Since it was a cipher and not a straight up announcement they probably got off lightly, too. If they needed to change synths or there was some kind of issue they wouldn't have started the crowdfund already. The only other synth it could be based off the Q&A would be ACE, but I haven't ever seen a crowdfund for that synth.
We have actually seen this before, for Koharu Rikka! During her crowdfund all TOKYO6 said at first was that she would receive a singing synth voice database if she met the final stretch goal, and they confirmed it was Synthesizer V later.
TLDR: Very likely they just weren't allowed to say she's going to SynthV yet. The actual question is if her box will be available after!