the full program is not downloaded - you get an installer 'stub' which downloads the full program during installation - and therefore you need to be connected to the internet during installation - and therefore this piece of advice Ensure you disable any security based software prior to reinstalling. is a really bad idea, unless the previously downloaded full program is cached somewhere on your computer so you can disable the internet connection whilst 'the securitity based software' is disabled, and they don't say anything about doing that

It would be better to review the security software settings, or even change program temporaly [using a trial maybe?]

Certainly review your security softwre settings as well as all the rest; I cannot believe that in this day and age companies are advising their users to disable any security software as if that were a matter of little consequence - what do they think it is for?

and if I am being pedantic I notice they don't even advise/tell you to re-enable it again immediately after installation - who writes this stuff I wonder? - not anyone bothered about security issues I would suggest... still it's not their computer at risk, is it