I found a solution to reading in this data was WMTS from geoserver:

library(leaflet)
library(dplyr)

leaflet() %>%
# Set initial map view
setView(lng = -4, lat = 57, zoom = 7) %>%
# Add WMS Tiles as an overlay group
addWMSTiles(
  baseUrl = "https://geo.spatialhub.scot/geoserver/ows?authkey=b85aa063-d598-4582-8e45-e7e6048718fc",
  layers = "ext_rpth:pub_rpth",
  options = WMSTileOptions(format = "image/png", transparent = TRUE),
  attribution = "XXX",
  group = "Paths"
)