I used the free Exiftool to examine the files (Exiftool is the most authoritative tool for examining the metadata of images). The file _P1M1575-spp6.jpg does contain a capture date:
[EXIF] Date/Time Original: 2014:07:02 16:32:07
However, the metadata structure is incorrect or has been corrupted; "exiftool -v" produces the following warnings:
Warning = Short directory size (missing 236415 bytes) | Warning = Bad IFD2 directory
This almost certainly is why LR can't read the capture date. Smells like a bug in SPP 6 that you should report to Sigma.