Seems that Synology packages use a slightly different naming strategy that simply cut -f2 -d_ is incorrect.

Retrieving list of available distributions
RELEASE="label":"Intel - 64-bit (DSM 6.0 or newer)","build":"linux-ubuntu-x86_64","distro":"synology","url":"https://downloads.plex.tv/plex-media-server/1.13.9.5456-ecd600442/PlexMediaServer-1.13.9.5456-ecd600442-x86_64.spk","checksum":"94df1233c217d0a0e13d892eba51a5caa999d7d0"
DOWNLOAD=https://downloads.plex.tv/plex-media-server/1.13.9.5456-ecd600442/PlexMediaServer-1.13.9.5456-ecd600442-x86_64.spk
CHECKSUM=94df1233c217d0a0e13d892eba51a5caa999d7d0
WARNING: Unable to detect installed version, first time?
INSTALLED_VERSION=
FILE_VERSION=64.spk
File already exists (PlexMediaServer-1.13.9.5456-ecd600442-x86_64.spk), won't download.

Whilst version checking isn't really used for non standard Linux OS builds, I think parseVersion could be modified to tokenize the version strings by - or _ first, and then looking for a token that matches /([0-9]+.?)+/ instead.