No, because leaving them in between the commands would leave the layers larger

No, because leaving them in between the commands would leave the layers larger

Wouldn't it be just one really long command, and hence it would be only 1 layer for the RUN command.

Let me show you a rough idea of what I mean:

Line 6 onwards:

ENV NODE_VERSION 12.18.2
ENV YARN_VERSION 1.22.4

RUN ARCH= && dpkgArch="$(dpkg --print-architecture)" \
.
.
.
 && apt-get update && apt-get install -y ca-certificates curl wget gnupg dirmngr xz-utils libatomic1 --no-install-recommends \
.
.
.

**(install node)**
&& find /usr/local -type f -executable -exec ldd '{}' ';' \
      | awk '/=>/ { print $(NF-1) }' \
      | sort -u \
      | xargs -r dpkg-query --search \
      | cut -d: -f1 \
      | sort -u \
      | xargs -r apt-mark manual \
**(test node)**
        && node --version \
        && npm --version
.
.
.

**(install yarn)**
&& find /usr/local -type f -executable -exec ldd '{}' ';' \
    | awk '/=>/ { print $(NF-1) }' \
    | sort -u \
    | xargs -r dpkg-query --search \
    | cut -d: -f1 \
    | sort -u \
    | xargs -r apt-mark manual \

**(test yarn)**
         && yarn --version

(finally purge all installed packages)
&& apt-get purge -y --auto-remove -o APT::AutoRemove::RecommendsImportant=false

The whole RUN command (which downloads packages, installs node, installs yarn, purges packages) should just create one single layer. There won't be any residual packages between the layers.

I think they were split originally to allow updating Yarn without rebuilding the whole image, but since Yarn 1 is in maintenance mode, I don't think that matters much anymore. You could try PR'ing something to see if the other maintainers agree.
You probably need to update the *.template files and then run the update.sh script to regenerate them