What is the format of Android M1 Initial Preview system.img?
wrong offset, partition start at sector 4096. you can use kpartx instead
newer android running dynamic partitions. the super partition can be unpacked with Dynamic Partition Tools
furthermore partitions are flagged as read-only, therefore mount -r flag is required
sudo -i

git clone https://github.com/AndroidDumps/Firmware_extractor.git
PATH=$PATH:$(realpath Firmware_extractor/tools/Linux/bin)

mkdir -p /mnt/android-1/system_root ../extracted
kpartx -av system.img
lpunpack -p system /dev/mapper/loop2p2 ../extracted

mount -t ext4 -o loop,ro,noexec,noload,noatime ../extracted/system.img /mnt/android-1/system_root
thunar /mnt/android-1/system_root

(or whatever kpartx tell you 2nd partition is mapped)

in case you need to make ext4-dedup/logical partition writeable, increase partition first, then duplicate shared blocks
cd ../extracted
dd if=/dev/zero bs=1G seek=1 count=0 of=system.img
resize2fs system.img 1G
e2fsck -E unshare_blocks system.img

you should be able to mount -w afterwards

re-packing requires new vbmeta signature with avbtool.py stored in first partition system.img1 start at sector 2048 (which goes too far here)
superrepack third-party cross-platform tool for easy unpack/repack system.img2 is recommended to automatize the whole re-packing process
you can however do it manually with lpmake (requires all partitions to be unpacked into ../extracted see above step lpunpack)
source
resize2fs -M system.img
e2fsck -yf system.img
lpmake --metadata-size 65536 --super-name super --metadata-slots 1 --device super:3229614080 --group main:2369171456 --partition system:readonly:787050496:main --image system=system.img --partition vendor:readonly:83619840:main --image vendor=vendor.img --partition product:readonly:1366814720:main --image product=product.img --partition system_ext:readonly:131686400:main --image system=system_ext.img --sparse --output super.img