[ninetyninenine]

The topic is about theoretical computer science which I would say is a math.

The authors last point is basically like what applied math is to math. It’s applied computer science.

[auggierose]

It is either "a kind of math", or "math", but not "a math".

[ninetyninenine]

Sounds ok to me in casual conversation. I use it like fruit. Orange is a fruit. Orange is also a kind of fruit. Orange is fruit doesn’t sound right though.

Empathetically speaking I’m sure it’s quite jarring for you when you read it.

[auggierose]

A single fruit

A single math

One of these expressions doesn't make sense. So no, you cannot use "math" like "fruit".

[ninetyninenine]

Ok, you are right. But as long as people understand my points I’m fine with the grammatical error.