Paul, IIRC Harman does coat for others, but they won't sell their papers under another label. ADOX Fine Print Variotone is coated by Harman, but this product has nothing to do with any Ilford product. It surely doesn't make sense to build a $50M coating plant, just because you want to produce some paper. OTOH, Harman pays Tetenal (and whoever else) to produce their chemicals. There can be mutual benefit just by that.
Which makes sense, there are some unique properties to some of the papers. If you can buy an Ilford paper for half the cost with a different name on it, why would you buy the Ilford paper? However if someone else wants you to make their paper using the same machinery, but maybe a different formula, there is nothing wrong with that, and good for Harmon to keep their plant operating at times they don't have product to make for themselves, rather then shuttering it and laying off all the staff.
As for chemicals, chemical plants are notoriously difficult to set up, and keep operating, environmental regulations change on almost a daily basis, you have inspections, and storage requirements, so if your volumes are not very high, then it can get too expensive to operate the plant. Chemicals often have special shipping regulations once they are manufactured, and import/export regulations on chemicals have only become much stricter in the last few years. So contracting with someone in Europe, someone else in North America and someone else in Asia makes sense.