That function has no stringsAsFactors argument, which is why you see that message. You can see this by reading the docs ?readr::read_csv or using the args function:
> args(readr::read_csv)
function (file, col_names = TRUE, col_types = NULL, locale = default_locale(),
na = c("", "NA"), quoted_na = TRUE, quote = "\"", comment = "",
trim_ws = TRUE, skip = 0, n_max = Inf, guess_max = min(1000,
n_max), progress = show_progress())
I haven't tested your example since I'm not clear on what you're expecting with factors, strings and numbers.