No, ARC-X will always save the entire media file to the archive. When you restore your work to FCPX you will have the entire media file to edit with again.
ARC-X will back-up any FCPX compatible video file type (3GP, AVI, MOV, MP4, MTS/M2TS, and MXF) created with any of the following codecs:
Apple Animation codec
Apple Intermediate codec
Apple ProRes (all versions)
Apple ProRes RAW and Apple ProRes RAW HQ
AVCHD (including AVCCAM, AVCHD Lite, and NXCAM)
AVC-ULTRA (including AVC-LongG, AVC-Intra Class 50/100/200/4:4:4, and AVC-Intra LT)
Canon Cinema RAW Light (requires the camera manufacturer’s plug-in software)
DV (including DVCAM, DVCPRO, and DVCPRO50)
DVCPRO HD
H.264
HDV
HEVC (requires macOS High Sierra 10.13)
iFrame
Motion JPEG (OpenDML only)
MPEG IMX (D-10)
QuickTime formats
REDCODE RAW (R3D) (requires the camera manufacturer’s plug-in software)
Uncompressed 8-bit 4:2:2
Uncompressed 10-bit 4:2:2
XAVC (including XAVC-S)
XDCAM HD/EX/HD422
XF-AVC
XF-HEVC
ARC-X can backup any of the following audio formats:
AAC
AIFF
BWF
CAF
MP3
MP4
RF64 (requires macOS High Sierra 10.13)
WAV
ARC-X can backup any of the following still-image formats:
BMP
GIF
HEIF (requires macOS High Sierra 10.13)
JPEG
PNG
PSD (static and layered)
RAW
TGA
TIFF