Lemond was gone after winning the 90 Tour, he wasn't competative thereafter. His decline, apart from the led poisoning, coincided with the rise of EPO use in the peloton. Indurain took off in 91, having gotten the boost. Perhaps he was already charged in 89-90, but it was still in the experimental phase and he was still working for Delgado.
This reads like the Indurain-EPO-train arrived in 91 and just plowed Lemond into dust. This is just plain wrong.
Just watch the Tour 91. Lemond was 3rd in the prologue, 3s behind Marie and 1s behind Breukink. He (and Breukink) took 1:40 in stage 1 on all other rivals. In the 73km time trial he was 2nd and only 8s slower than Indurain.
He lost time in the TTT before and he had a weak team, sure. But he lead the tour with >2 min on Indurain.
On the first mountain stage, he said that he felt good physically and complained that Indurain and Delgado did not work on reeling in the Escape. But why should they? Lemond lost the yellow to Leblanc but said, that he was not worried.
Well the tour-defining second mountain stage to Val Louron:
Lemond attacked on the 3rd climb from the finish (Tourmalet), but was reeled in and dropped (18s behind the group with Indurain, Bugno, Chiapucci on the top). It was in the descent when Indurain noted that Lemond was missing, that Indurain attacked. Indurain was a great descender and gained some time while Lemond joined the group again. But there was no real cooperation, which lead to Chiapucci escaping, and joining Indurain (who waited).
At the feeding point, Bugno attacked, but Lemond caught him. Maybe this hindered Lemond to eat properly and lead to his collapse.
The group with Lemond, Bugno, Hampsten, Leblanc, Mottet and Chosas was not cooperating at all which can be easily proven by the point that Fignon caught the group after being 1 minute behind at the top of the Tourmalet and instantly went to the front and pulled. This got the group back a bit and Wikipedia even claims that they were within sight of Indurain and Chiapucci, but this can not be comfirmed by the broadcast.
Well, Lemond did pull eventually - or better - he tried to attack repeatedly and simply bonked at the Aspin and Val Louron. I think this was more psychologically than physically. He was dropped by all other members of the group except for Leblanc and was even caught by his teammate Eric Boyer. Bugno meanwhile reached the finish only 1:30 after Chiapucci. Fignon was 4th. This 1:30 gap shows that Indurain and Chiapucci were not just much much stronger than the rest. They just profited from the group 2 dynamics.
Lemond road bad tactically on that day and had not enough help from others. And after a while he just faltered.
Remind you that this was not that unexpected, because even in 89 and 90 Lemond was not really physically stronger than his rivals. And he was very lucky in both editions. E.g. in 89 in the stage 18 stage, Fignon escaped. Lemond was behind with Rooks, Lejarreta, Kelly and Theunisse. Several of them did big pulls and helped him keep the gap small even though they should have had no interest in doing so. In 1990 Chiapucci did his famous suicide attack while in yellow that lost him the tour. If he just followed Lemond, he would not have lost so much time.
Lemond had weak teams and relied on riding strategically smart, getting help from other riders / teams and being a better time trialist. It just did not work in 1991. And in the remaining 2 hard days, he rode like a madman, doing nonsensical attacks. But he could not drop the others, well he seldom could do this even in 89 and 90.
If course I do not claim that EPO had no effect on the results or that the riders were clean. Of course not. It was just a lot more nuanced than you wrote.