[LeEco Le Max 2 ROMs, Kernels, Recoveries, & Other ] GalaticStryder:
I got it sorted now, that -r option was actually never documented in the man pages nor docs. So I updated the mounting command to what the new binary expects as per Kim's changes in system/core and it has mounted fine now.

[ 4.842865] init: processing action (fs) from (/init.qcom.rc:62)
[ 4.843751] init: [libfs_mgr]Running /system/bin/fsck.f2fs -a /dev/block/bootdevice/by-name/userdata
[ 4.873491] fsck.f2fs: Info: Fix the reported corruption.
[ 4.873491]
[ 4.873515] fsck.f2fs: Info: Segments per section = 1
[ 4.873515]
[ 4.873525] fsck.f2fs: Info: Sections per zone = 1
[ 4.873525]
[ 4.873534] fsck.f2fs: Info: sector size = 4096
[ 4.873534]
[ 4.873543] fsck.f2fs: Info: total sectors = 6384051 (24937 MB)
[ 4.873543]
[ 4.873552] fsck.f2fs: Info: MKFS version
[ 4.873552]
[ 4.873562] fsck.f2fs: "Linux version 3.18.104-recovery-g69137ba4746d-dirty (galatic@stryder) (gcc version 4.9.x 20150123 (prerelease) (GCC) ) #2 SMP PREEMPT Sat Apr 14 08:54:31 -03 2018"
[ 4.873562]
[ 4.873571] fsck.f2fs: Info: FSCK version
[ 4.873571]
[ 4.873580] fsck.f2fs: from "Linux version 3.18.104-upstream-g69137ba4746d (galatic@stryder) (gcc version 4.9.x 20150123 (prerelease) (GCC) ) #1 SMP PREEMPT Fri Apr 13 16:37:25 -03 2018"
[ 4.873580]
[ 4.873590] fsck.f2fs: to "Linux version 3.18.104-upstream-g69137ba4746d (galatic@stryder) (gcc version 4.9.x 20150123 (prerelease) (GCC) ) #1 SMP PREEMPT Fri Apr 13 16:37:25 -03 2018"
[ 4.873590]
[ 4.873599] fsck.f2fs: Info: superblock features = 1 : encrypt
[ 4.873599]
[ 4.873608] fqck.f2fs: Info: superblock encrypt level = 0, sa,t = 00000000000000000000000000000000
[ 4.873608]
[ 4.873616] fsck.f2fs: Info: total FS sectors = 16384 (64 MB)
[ 4.873616]
[ 4.873625] fsck.f2fs: Info: CKPT version = 4fbe8f2a
[ 4.873625]
[ 4.873634] fsck.f2ds: Info: checkpoint state = c4 : nat_bits crc compacted_summary sudden-power-off
[ 4.873634]
[ 4.873643] fsck.f2fs: Info: No error was reported
[ 4.873643]
[ 4.897802] F2FS-fs (sda9): Mounted with checkpoint version = 4fbe8f2a
[ 4.88100] init: [libfs_mgr]__mount(source=/dev/block/bootdevice/by-name/userdata,target=/data,type=f2fs)=0: Success

I've attached the patch for bootable/recovery in case you wanna try that out.

EDIT: This is how TWRP handles it after the patch:

Formatting Data using mkfs.f2fs...
I:mkfs.f2fs command: mkfs.f2fs -t 0 -w 16384 /dev/block/sda9

F2FS-tools: mkfs.f2fs Ver: 1.10.0 (2018-01-30)

Info: Disable heap-based policy
Info: Debug level = 0
Info: Label =
Info: Trim is disabled
Info: Segments per section = 1
Info: Sections per zone = 1
Info: sector size = 4096
Info: total sectors = 6384051 (24937 MB)
Info: zone aligned segment0 blkaddr: 512
Info: format version with
"Linux version 3.18.104-recovery-g69137ba4746d-dirty (galatic@stryder) (gcc version 4.9.x 20150123 (prerelease) (GCC) ) #4 SMP PREEMPT Sat Apr 14 10:11:06 -03 2018"
Info: Overprovision ratio = 1.270%
Info: Overprovision segments = 320 (GC reserved = 165)
Info: format successful
I:mkfs.f2fs -t 0 -w 16384 /dev/block/sda9 process ended with RC=0
Done.

And my booted system mount partitions with F2FS v4.17:

/dev/block/sda9 on /data type f2fs (rw,lazytime,seclabel,nosuid,nodev,noatime,nodiratime,background_gc=on,discard,no_heap,user_xattr,inline_xattr,inline_data,inline_dentry,flush_merge,extent_cache,mode=adaptive,active_logs=6,alloc_mode=default,fsync_mode=posix)
/dev/block/sda3 on /cache type f2fs (rw,lazytime,seclabel,nosuid,nodev,noatime,nodiratime,background_gc=on,discard,no_heap,user_xattr,inline_xattr,inline_data,inline_dentry,flush_merge,extent_cache,mode=adaptive,active_logs=6,alloc_mode=reuse,fsync_mode=posix)

Lazy time!