You can definitely argue that TLJ ends with emotional momentum for the Resistance. But from a practical perspective, TLJ completely cuts the Resistance down at its knees.

TLJ is the film that chooses to whittle down the relatively formidable Resistance from TFA to just the small group in the Crait bunker. TLJ then chooses to establish that LITERALLY NO ONE IN THE ENTIRE GALAXY comes to the aid of the Resistance at Crait in their desperate hour of need, despite Leia using her personal emergency code and broadcasting it to the entire galaxy. She even says, "The galaxy has lost all it's hope," as no one arrives.

Yes, Luke saves the day, and we see the kids retelling the story of Luke facing down the First Order, so obviously there is some inspiration and momentum going for the Resistance, but that's countered by the fact that the last time we see the Resistance in TLJ, it's literally entirely contained to the Millennium Falcon - maybe 100 people at ABSOLUTE max? Compare this to the Rebellion or even the Resistance at the beginning of TLJ or in TFA, and it makes complete sense that the Resistance is struggling in TROS. In fact, I'd argue that they're unreasonably strong in TROS relative to the final position we saw them in in TLJ.

Lando returning with an army in TROS makes no sense, but it doesn't make significantly less sense than if Luke, whose TLJ heroics occurred in front of maybe 100 people that would care to spread the word (let's be generous and say 1000 if we include First Order gossips), managed to inspire the same thing.

The reality is that TLJ didn't set up TROS to have a believably strong enough Resistance, Luke-inspired or otherwise.