Is that right to add inhibitor in boiler filter?

I had an engineer today added inhibitor in the boiler filter via Magnaclean when he did the boiler service after taking off the filter.

But before the engineers all added inhibitor in the radiators, and I searched online that majority information shows the inhibitor should be added in radiator.
So is it ok or right to add inhibitor in the boiler filter via Magnaclean?

Thanks for any input!

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