I got a terse one sentence from Garmin, " We don't recommend...". It sounds like a CYA statement with no allowance for a capable electrical splice. Yes, it is that clamp on mount. My concern with the large holes is weakening the guard.
I do avoid splices in data cables in my job as a networkk admin. I am not so sure there is a problem with them in these applications. Knowing that the installs you did involved splices is encouraging. Did you solder joints?