Not speaking for others, but having fringe groups as security during official business, doesn't speak well of what other uses there might be for such groups.
That's fairly speculative for an explicit declaration that Trump is a white nationalist, and very much guilt-by-association.
Also, I don't really consider a "white nationalist" to be a "nationalist" by the usual sense of the word (applying to nation), but rather a rarer, more abstract sense of the word (applying abstractly to "groups" such as race). White nationalism isn't necessarily compatible with race-defined nationalism e.g. if you would prioritise foreign whites over domestic non-whites.
It must be speculative, that's the entire purpose of associating with such groups and using them as political ammunition. Doesn't stop these people from throwing them under the bus later, if the bet doesn't pan out.