R Highcharter: Changing xAxis Type from datetime to category

I want to change the x-axis type from datetime to category. For example: Primary Chart would be a line chart by group and drilldown chart would be a column chart.

There is a solution given in Highcharts (Highcharts, Can you change the chart type for drilldowns?, Highcharts chart with 'datetime' xAxis - use categories on drilldown) but I'm not able to translate it to R.

PFB the code.

library("dplyr")
library("purrr")
library("highcharter")

df <- data_frame(
  car_brand = c("Hyundai","Hyundai","Hyundai", "Benz","Benz","Benz", "Tesla","Tesla","Tesla"),
  units_sold = c(10,15,20,11,8,13,6,5,7),
  date = c("2019-01-01", "2019-02-01","2019-03-01","2019-01-01","2019-02-01","2019-03-01","2019-01-01","2019-02-01","2019-03-01")
)

df$units_sold <- as.numeric(df$units_sold)
df$date <- as.Date(df$date)
df$drilldown <- paste(df$car_brand, ",", df$date)
carBrands<- df %>%
  select(date, car_brand)

getCarDetails<- function(brands){

  carList <- list()
  listofdfs <- list() #Create a list in which you intend to save your df's.

  for(i in 1:nrow(brands)){ #Loop through the numbers of ID's instead of the ID's

    #You are going to use games[i] instead of i to get the ID
    BrandCarData <- data_frame(
      car = c("H1","H2","H3","H4","H5"),
      units = c(1,2,3,4,5)
    )
    BrandCarData$units <- as.numeric(BrandCarData$units)
    dsCar <- list_parse2(BrandCarData)
    listofdfs[[i]] <- dsCar
    carList[[i]] <- list (name = brands[[2]][i],
                          type = "column",
                          id = paste(brands[[2]][i], ",", brands[[1]][i]),
                          data = listofdfs[[i]])
  }

  return(carList) #Return the list of dataframes.
}

listCar <- getCarDetails(brands = carBrands)

dfDates <- NULL
dfDates$Date <- datetime_to_timestamp(as.Date(df$date, format = "%Y-%m-%d"))
hc <- hchart(df,"line", hcaes(x=date, y =
                                units_sold, group
                              = car_brand )) %>%
  hc_xAxis(categories = dfDates$Date, title = list(text = "<b>Date</b>"), type = "datetime") %>%
  hc_plotOptions(column = list(dataLabels = list(enabled = FALSE), enableMouseTracking = TRUE))%>%
  hc_tooltip(borderWidth = 1.5,
             pointFormat = paste('<span style="color:{series.color}">{series.name}</span>: <b>{point.y}</b><br/>')) %>%
  hc_legend(enabled = TRUE) %>%
  hc_title(text = "Brand Units Sold Monthy Trend",
           style = list(fontSize = "12px", fontWeight = "bold")) %>%
  hc_yAxis(title = list(text = "<b>Units <br>(In Thousands)</br></b>"))
hc

hc1 <- hc %>%
  hc_drilldown(allowPointDrilldown = TRUE,
               series = listCar)
hc1

As can be seen from the attached graphs, the x-axis of second chart is still showing x-axis labels in datetime format.

Any suggestions?