That wouldn’t surprise me at all - particularly as at V1.0 of a rewrite. Achieving compatibility with a former look may be something that is only arrived at over time i.e. incrementally over the next 3 years.
I still look forward to
an adaptable and extremely customizable user interface
Within limits that should keep everybody happy - but it is an objective over 3 years.
In principle, this is a good thing i.e. Thunderbird will still work on the desktop/laptop but it will work better than it currently does on a touchscreen tablet and better than it currently does on a smartphone. (They also observe that they want to make Thunderbird fully compatible with current accessibility technology, which was probably not such a requirement, or even a possibility, back in the stone age when Thunderbird started.)
I’m pretty confident that Rome won’t be built in a day. 
I’m not disagreeing with that claim but where specifically did they say that?
It is perhaps implied by “extremely customizable”.