Don't ever apologies for providing advice, that after all is why we are all here and the great thing about advice is that the user can choose what to do with it. In this case I shall be applying it to the letter.
If it really is just 100-150 old topics, then an nginx map makes even more sense.
I can see it from your point of view, that it's easy for NodeBB to do, although I urge you to consider that nginx is very good at what it does, and so having nginx check for an old url to redirect is quite simple.
For example, if the old forum had topics using this url: /t/<topic id>, then you could write a location block that only responded to that folder, and applied the map to redirect the user based on those topic IDs.