Firstly, note that the Maharal and Maharsha (and I would assume others) consider this Gemara to be Agadic and not Halachic, as they put it in their Chiddushei Agados. Thus, it could very likely that this is a plain Issur, and this Gemara just adds "Agadic warning" to this Aveirah.
Secondly, the Poskim (Shulchan Aruch, Aruch Hashulchan EH 23:3) seem to quote the wording of the Rambam that it is Assur, so "it is forbidden".
Thirdly, there are other opinions in the Gemara there that consider such a person an "Avaryan", which clearly indicates that it is an Aveirah, and that could be the source of Rambam. (h/t to Fred)
Fourthly, the major source of discussion is Ran to Rif Shabbos 39b, who says:
אלא ודאי לישנא דיהא בנדוי היינו לומר שראוי לנדותו והיינו דפרכינן הכא ולימא אסור כלומר ובהכי משמע שראוי לנדותו שכך הוא המדה בכל האיסורים שהעובר עליהן ראוי לנדותו ופרקינן דנקט כי האי לישנא לאשמועינן שאיסורו יותר מפני שמגרה יצה"ר בעצמו:
The "Chavrusa" Gemara and Shu"t Amudei Eish understands this Ran that it is more Chamur to say Nidui than to say Assur, and that Issur is included in Nidui, nit that it is only a Niddui and No Issur. See also Aruch Laner and Chasam Sofer there who learn similarly (although the Aruch Laner has a bit of a twist), h/t to commenters who pointed them out.
Fifthly, see here for a very strange interpretation of what the phrase "Veleima Assur" means והדבר פלא.
Sixthly, there is a huge discussion of how Rambam understands and applies this Gemara. See Issurei Biah 21:19, Talmud Torah 6:14, and a boatload of Mefarshim in those places, as well as here in Niddah, who discuss his Shitah. Some (see Divrei Chamudos here in Niddah and others) say that he doesn't even consider someone to be in Nidui unless they are actually Motzi Zera Levatalah, although I believe that everyone holds that it is Assur.