The nodejs-devel does not include npm whereas both nodejs10-devel and nodejs12-devel do include npm.
A version of npm is available in the devel image, although not one from nodesource.
root@fa72f308944c:/# apt-cache madison npm npm | 5.8.0+ds6-2~bpo9+1 | http://mirrors.wikimedia.org/debian stretch-backports/main amd64 Packages
Was npm not added deliberately for any reason?