I don't really consider it a gimmick. It's an enticement to use their platform - and this seems to be a common thing with their competitors now too - like the recently announced Trading 212 card.
Motivation aside, there are a lot of positives here for consumers - the card is actually very good - unlimited FX including foreign cash withdrawals. And while it's not real cashback, you could choose to invest in the XEON ETF, let it build up for a while, earning its 3.99% return and then cash out.
Optimal use of TR is to max out the 50K cash earning interest. Then have your monthly spending cash on top of that. Then spend it on the card, earning the saveback, and do your normal ETF investing with them as a destination for the saveback.
It's not for everyone, and of course they make money when you trade (don't all brokers?).
Biggest issue for me, and I won't use it until they do this - is that you can't set a low spending limit or otherwise separate your bulk cash.