Since this block doesn't seem to exist for any other Cloudflare guarded site for this major ISP that I can see or hear reports of, FT seems either irrationally paranoid or uncaring about this segment uniquely from other major sites.

It is like blocking all Comcast IPs because just the sheer number of subscribers guarantee a substantial number of hacking/spam attempts via those IPs.

The only such broad blocks I know of are for AWS IPs because those can be and have been used anonymously to spam/hack.

Banning the largest ISP of a country is not a rational choice.