TER-GV (near Lille) and NRT ticket

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I'm planning to use TER-GV trains between Dunkerque and Lille:

Two inobvious things

-many sources, including timetable booklette downlodable from sncf website, for example this one or this one page of ter-sncf website states, that High-speed supplement is needed for usage of TER-GV. At the same time - purchase module of SNCF website shows the same price (15,30) to all TER services between Dunkerkque and Lille (without any special mark on TER-GV services, all them are marked as just TER, including those, which has travel time about half hour instead hour and no stops and arrival to Lille-Europe instead of Flandes - those obvously are TER-GV). Is this high-speed supplement really needed?

-If the answer to previous question is "yes" - my 2nd question - if this TER-GV supplement is needed in case of usage NRT ticket (purchased in Germany)

Is this high-speed supplement really needed?

Yes, it is compulsory. Apparently it's the problem with the module/website, it simply doesn't show the difference (confirmed by Q&A on SNCF website)

If the answer to previous question is "yes" - my 2nd question - if this TER-GV supplement is needed in case of usage NRT ticket (purchased in Germany)

In the context of supplement NRT ticket isn't different from base TER tariff, so unlike in some other countries "international" ticket won't save you from paying the supplement. Although there is a minor chance that French conductor won't sort out things with the unusual ticket and just let you travel without supplement, but it's kind of risky to rely on such possibility, imho. Much easier is just to pay extra 3 euro

this supllmt ticket is valid 1 day, not 1 ride
it is pretty common that some trains have surcharges, also for TCV/SCIC-ticketholders.
in SUMMER this region often has many more special promotions, only posted in french on ter-sncf.com, and these may well work out  loads cheaper as that SCIC.