He was probably moreso frustrated that he was being forced to adapt to the letter a story riddled with nonsensical and repetitive events, plotholes, padding and poorly written characters.
His role as the director was to deliver a cohesive story that the viewers would be able to understand and be entertained by hence his desire to develop the Animamates as characters (and probably some other stuff) but again considering Toei gave him extra budget, there's no reason for them to refuse his desire to develop the plot
especially after he was vocal about it.
This is the Japanese media, everything they say is super filtered so if Toei is allowing him to say this, well ya know
Apparently at one point it got so difficult that he wanted out of the project and simply not do it (but of course he couldn't do so).
Why do you think the only one who was allowed to get extra scenes was Cosmos, aka Takeuchi's natural favorite after UsaMe?
The same thing happened with Kon and the finale of Eternal.
During the interview, she was trying as hard to say between the lines, ''Look I know the plot is dumb, I wanted to change it but I couldn't. I'm sorry, it wasn't within my power, I tried my best. Please don't send me death threats if you dislike the movie''.
We also know that both Kon and Takahashi never got to communicate with the Hime but she was filtering their questions by Osabu which I have to ask, then why in the world would she be chief supervisor on something she simply doesn't want to do?