There were a lot of people at the exhibition — more than they came last year.

网友的这个句子确实有问题。问题就出在 more than 后面误加了代词 they。

在柯帕斯英语网有过很多类似的讨论，在比较分句中不能出现“比较对应项”。你的句子中的 more than...其实是 more (people) than 的省略说法，网友句子中的 they 就属于比较对应项。“对应”谁？对应 more people。改正的办法是去掉 they：

There were a lot of people at the exhibition — more than came last year. 展览会上来了许多人——比去年来的人要更多。