I do agree with you, Nadal fans do fear the threat of Djokovic here.
But this is also the place where Nadal fans should feel the most confident. Nadal is still the man here in Paris, even if Djokovic owns the rest of the world. Nadal also has a better chance of getting to final and also with significantly less wear and tear on the body, which will help him.
At AO19 the way Djokovic won SF pretty much defined state of mind for both players when they started playing final - Djokovic was in his absolute killing mode, Nadal was truly worried/scared of what he saw in SF Djokovic played.
Therefore, the crucial moment before they meet in RG19 finals is the way Thiem vs Djokovic SF unfolds., obviously if both Djokovic and Thiem reach SF. A straight sets no tiebreaks win by Djokovic, and without a single 5 set match during 2 weeks (maybe 1 or 2 sets lost, without Djokovic struggling to win any of the matches before final) brings their chance to exact 50/50, but with slight mental advantage for Djokovic which is all he needs to win it in 4 or 5, and everyone knows that.
Surely, I am aware PC is Nadal's playground and that is reflected in 50/50 estimate. But OTOH, if Djokovic hardly wins against Thiem and/or gets worn out in another 5 setter (QF?) during tournament, Nadal will win RG19 comfortably. After Madrid and Rome, RG winner will be decided in a match of a mind against a mind. It always was, when both are playing close to their maximum at that moment.
If both play their best/optimal game during tournament, there is almost no chance of a straight sets win at RG19 finals for neither Nadal nor Djokovic. Whether it will be another match of them 2 to remember, we shall see.