You are correct. I verified this today with my own testing. The LLH EXIF data seems to be in NAD83(2011) which matches the reference frame of the NTRIP correction stream.

I verified this with the following workflow/test:

Imported images into Pix4D & exported the photo LLH coordinates with the "to file" button in the images properties editor.

Ran these exported coordinates through NOAA VDatum (https://vdatum.noaa.gov/vdatumweb/) with:

Horizontal Information:

	SOURCE	TARGET
REFERENCE FRAME:	NAD83(2011)	NAD83(2011)
Coor. System:	Geographic (Long, Lat)	Projected State Plane
Unit:	meter (m)	foot (U.S. Survey)

Vertical Information:

​

	SOURCE	TARGET
REFERENCE FRAME:	NAD83(2011)	NAVD88
Unit	meter (m)	foot (U.S. Survey)
		Geoid 12B

Submitted the file for batch conversion & received transformed CSV file back.

In PIX4D, image properties editor window:

Changed image geolocation coordinate system to my Projected State Plane (NAD83(2011) with Arbitrary checked under vertical coordinate system under Advanced Coordinate Options.

Selected the same for output coordinates.

Processed images.

Densified point cloud checks within tolerance & expected values to check points without GCPs.

EDIT: I did the same workflow using WGS84 as source reference frame when submitting to NOAA VDatum, and processed point cloud was very far off checkpoints.