I think they were referring to back-end part of a application or complete applications that end up as a back-end piece (SQL, HTTP server, etc) of a more user facing application. This is generally where the amount of state easily manageable and well defined.
Front end testing is much harder as you have quite a bit of state you need to manage, and things like "Is this button visible to a user" are hard for a computer to answer as for a computer you need to render the entire page, then use machine vision to look for the button and verify the text is a readable size (not a cheap operation). In the front end, you can't get away with only rendering part of it, since anything could trigger a modal/overlay, or cause some z-order/clipping/scaling issue.
Front end testing is much harder as you have quite a bit of state you need to manage, and things like "Is this button visible to a user" are hard for a computer to answer as for a computer you need to render the entire page, then use machine vision to look for the button and verify the text is a readable size (not a cheap operation). In the front end, you can't get away with only rendering part of it, since anything could trigger a modal/overlay, or cause some z-order/clipping/scaling issue.