It's also a leaded solder, which is much easier to get nice joints from than lead-free.
Flux is great, adding some before you solder is a good tip, and will likely help you make better joints if as you said, you haven't perfectly cleaned everything. The only downside is perhaps some more cleanup afterwards. There's no reason to disparage it's use.
0: https://www.felder.de/products/electronic-applications/manua...
Rework... good bloody luck.
Fluxless rework isn't difficult at all. I do it dozens of times a day.
Production reflow without extra added flux is normal. It's the goal for every assembly line anywhere and is almost always achieved.
Rework of existing joints after assembly without flux is just about impossible. The oxidation of the joints guarantees it. Many times the flux in the paste or wire is sufficient, but this is more true for through-hole than for surface-mount, mainly due to joint geometry and available volumes. If you are regularly reworking old fine-pitch surface mount joints previously assembled with no-clean flux, without flux of your own... then I'd like to know what materials you're using, because that is not possible in general. The "no-clean" fluxes are notorious for causing rework trouble, and they are difficult to clean off once reflow temperatures have been achieved (that's why we call them "can't-clean"). They are never active twice, so they must be removed and replaced once heated the first time. We have to use special solvent flux removers around here (based on DuPont's hellish Vertrel XF) to get the crap off just to be able to access joints again. (This is on assembly work done out-of-house or on COTS devices, made with who-knows-what.)
Nope, we don't use flux at all for rework. As the majority of our boards are solid metal, we just slap them on a heater and poke the components down or slide them into place. Flux isn't needed. Even with crappy RoHS tin solder. If you're getting significant-enough oxidation that interferes with rework with the product fresh out of the oven, or even a day or two later, you're doing something extremely wrong. Check and make sure your production paste isn't contaminated.
I really don't think either's wrong, and anyway I assume 'production engineering' is not using how-to-solder tutorials.
That is part of production engineering which is mostly about making sure that your product doesn't come back again because it's broken.
It's a different world from a hobbyist's one-off project. This advice was never intended for you, and appropriate advice can differ between audiences.