Or Roman Catholic?
Why, indeed?
I prefer to be non-demoninational.
No need to subscribe to anyone else's version of Christianity.
I think the author is using the term "Protestant" in this context of his beliefs lining up with those held by the historical early church and then continued in the Evangelical Protestant churches rather than those put forward as necessary for salvation by modern Roman Catholicism. He also rules out all the heretical modern Protestant denominations that have strayed from the ancient rule of faith.
As for "subscribing" to another's version of Christianity, I think it is a good thing to fellowship with those to whom we share the common faith and we are encouraged to do that in Scripture. There are many, many doctrines of the Christian faith that have always been believed everywhere and by all and we can know what these are because they are taught to us plainly in Scripture.