Congratulations on your new setup.
The more you think about it, the more absurd the excuse is.
As far as I can tell, he made sure all his PON stick work. There is no need for his customer to call TM.
Those who go out and buy their own PON stick, flash it or do whatever they want, don't need to call TM either. Their ONU will surely works.
The question is... Why is TM so sure it is his PON stick? So many people are selling them online.

Let's say someone really call 100 and talk about PON stick, I am sure their response is the same as people using their own router: Not officially supported.
None of them requires escalation and involvement of legal team.

From my experience dealing with call center folks, they don't care either. Their KPI is how many calls they answered per day. Just say it is not supported, hang up = KPI + 1.

Like I said before, have two side (two team),
one is allow as long setup is correct and customer are happy
one is dislike as make their work harder

I did call it after OLT vendor called, I told him not to sell my name just solve the issue,
because calling 100, it involved multiple party and I willing swap his PON Stick just for solve Technician and Vendor issue and clear my name on TM
still, I haven't received his PON Stick yet (I guess he using Black D-Link?, no need to use PON Stick)

vendor said is clear, OLT cannot accept and whitelist the PON Stick

we have discussed, most of them insist to support of selling PON Stick as long OLT not making any false Alarm and got few friend who work at TM can help me to assist when issue come

as long as customer is happy with their fancy equipment, they don't mind.

if PON stick issue, I can ask my friend to check customer via PPPoE Id or PLOAM Password what is the issue so I can fix and prepare OMCI,
basically outside of official help, but I still can try make it work

OLT Vendor Id 0x00 can be FHTT (Fiberhome) or other brand, mostly 100% is FHTT I facing behaving odd issue, still can solve by adjusting OMCI.ini file

If happening again, I got my backup plan.

Please check out the "Ditch ONU, use GPON SFP on Business Grade Router" Topic

wow, can you access WebGUI and download the XML file? and edit to enable telnet or ssh like this
https://github.com/Anime4000/Hacking_Huawei_HG8240H5_ONT

if you managed login ssh/telnet, do this:
omcicmd mib show meid 131

this tell what OLT are you on

try login WebGUI while fiber is connected