I think the issue might be that the version of FFmpeg Tools in the CoreELEC nightly addons Repo just hasn’t been built with the --enable-libx265 flag, or even the --enable-libx264 flag as that encoder also does not work.
https://trac.ffmpeg.org/wiki/Encode/H.265
ffmpeg version 4.2.1 Copyright (c) 2000-2019 the FFmpeg developers
built with gcc 8.3.0 (GCC)
configuration: --enable-ffmpeg --disable-ffplay --enable-ffprobe --enable-static --disable-shared --enable-gpl --disable-doc --enable-avresample --disable-lzma --disable-alsa --arch=arm --cpu=cortex-a53 --cross-prefix=/var/corebuild/CoreELEC/build.CoreELEC-Amlogic.arm-9.2-devel/toolchain/bin/armv8a-libreelec-linux-gnueabi- --enable-cross-compile --sysroot=/var/corebuild/CoreELEC/build.CoreELEC-Amlogic.arm-9.2-devel/toolchain/armv8a-libreelec-linux-gnueabi/sysroot --sysinclude=/var/corebuild/CoreELEC/build.CoreELEC-Amlogic.arm-9.2-devel/toolchain/armv8a-libreelec-linux-gnueabi/sysroot/usr/include --target-os=linux --nm=/var/corebuild/CoreELEC/build.CoreELEC-Amlogic.arm-9.2-devel/toolchain/bin/armv8a-libreelec-linux-gnueabi-nm --ar=/var/corebuild/CoreELEC/build.CoreELEC-Amlogic.arm-9.2-devel/toolchain/bin/armv8a-libreelec-linux-gnueabi-ar --as=/var/corebuild/CoreELEC/build.CoreELEC-Amlogic.arm-9.2-devel/toolchain/bin/armv8a-libreelec-linux-gnueabi-gcc --cc=/var/corebuild/CoreELEC/build.CoreELEC-Amlogic.arm-9.2-devel/toolchain/bin/armv8a-libreelec-linux-gnueabi-gcc --ld=/var/corebuild/CoreELEC/build.CoreELEC-Amlogic.arm-9.2-devel/toolchain/bin/armv8a-libreelec-linux-gnueabi-gcc --pkg-config=/var/corebuild/CoreELEC/build.CoreELEC-Amlogic.arm-9.2-devel/toolchain/bin/pkg-config --host-cc=/var/corebuild/CoreELEC/build.CoreELEC-Amlogic.arm-9.2-devel/toolchain/bin/host-gcc --host-cflags='-march=native -O2 -Wall -pipe -I/var/corebuild/CoreELEC/build.CoreELEC-Amlogic.arm-9.2-devel/toolchain/include -Wno-format-security' --host-ldflags='-Wl,-rpath,/var/corebuild/CoreELEC/build.CoreELEC-Amlogic.arm-9.2-devel/toolchain/lib -L/var/corebuild/CoreELEC/build.CoreELEC-Amlogic.arm-9.2-devel/toolchain/lib' --host-extralibs=-lm --extra-cflags='-march=armv8-a+crc -mtune=cortex-a53 -mabi=aapcs-linux -Wno-psabi -Wa,-mno-warn-deprecated -mcpu=cortex-a53 -mfloat-abi=hard -mfpu=neon-fp-armv8 -fomit-frame-pointer -Wall -pipe -Os -I/var/corebuild/CoreELEC/build.CoreELEC-Amlogic.arm-9.2-devel/gnutls-3.6.10/.INSTALL_PKG/usr/include' --extra-ldflags='-march=armv8-a+crc -mtune=cortex-a53 -Wl,--as-needed -L/var/corebuild/CoreELEC/build.CoreELEC-Amlogic.arm-9.2-devel/gnutls-3.6.10/.INSTALL_PKG/usr/lib' --extra-libs= --enable-pic --enable-gnutls --disable-openssl --disable-hardcoded-tables
libavutil 56. 31.100 / 56. 31.100
libavcodec 58. 54.100 / 58. 54.100
libavformat 58. 29.100 / 58. 29.100
libavdevice 58. 8.100 / 58. 8.100
libavfilter 7. 57.100 / 7. 57.100
libavresample 4. 0. 0 / 4. 0. 0
libswscale 5. 5.100 / 5. 5.100
libswresample 3. 5.100 / 3. 5.100
libpostproc 55. 5.100 / 55. 5.100
Hyper fast Audio and Video encoder
usage: ffmpeg [options] [[infile options] -i infile]... {[outfile options] outfile}...