I guess it is far to early for negative conclusions. They stated that there will be developer tools but maybe not from the start, meaning the release in February. It would be nuts to spend energy on making them ready for release when they are struggling with performance optimisation. One thing at a time.
A lot depends on whether the Beta stops working for good after the 12 days or if you can still start it, if offline, maybe. In the latter case, I would not be surprise if someone finds a way to smuggling stuff into TSW scenes that was developed using the standard UE4 development tools. There is just too much time between the official end of the beta and the official release for people to sit and do nothing.
But even failing that, I am sure there will be developer support some time in spring. They had this recent press release on setting up a hobbyist asset swapping place outside Steam, so they will make sure people find it easy to contribute. Whether that be reskins, I don't know. I see the developments in 3D modelling quite adverse to reskinning which best works on simple structures. However, developers of rolling stock might start to actively support reskinning by adding a decal on top of the highly sophisticated materials cook-up that represents the model skin.