This is just a follow up. It was a repository having incompatible versions of packages for a moment due to gcc-10-base rebuild.
arm64 had version 10.2.0-18 but amd64 and i386 had version 10.2.0-17. So it was not possible to install them at the same time. They are now in sync, and installing libraries and binaries from arm64 on amd64 works. They execute correctly too. user@debian:~$ uname -m x86_64 user@debian:~$ file /usr/bin/zstd /usr/bin/zstd: ELF 64-bit LSB pie executable, ARM aarch64, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux-aarch64.so.1, BuildID[sha1]=f7ae7d8114094829a26bb4f24ac992d58df1c5bb, for GNU/Linux 3.7.0, stripped user@debian:~$ /usr/bin/zstd -b /var/log/Xorg.0.log 3#Xorg.0.log : 78787 -> 10456 (7.535), 157.2 MB/s , 355.0 MB/s user@debian:~$ Nice!