Your source exactly cites what rules it violates. You need only go to the citation to find it, where it states:
"Grammatical constraints impose diverse requirements on the relations
between words and phrases in a sentence... Grammatical constraints
impose many structural and featural requirements on the relations
between words and phrases in a sentence, which include constraints on
anaphora, agreement, case, and unbounded dependencies..."
-"Grammatical Illusions and Selective Fallibility in Real-time Language Comprehension" by Colin Philips, Matthew W. Wagers and
Ell F. Lau
Your example includes an anaphora, which device is not innately ungrammatical, but that anaphora is an example of an ungrammatical unbounded dependency in that the subordinate "than" clause's subject "I" in what is a parallel construction lacks agreement with the parallel main clause's subject "more people." That's because the action of the subject "more people" is "have gone to Russia," which substantively lacks agreement with the subordinate clause's anaphora, the extension of which is "than I (have been to Russia)." "More people have been to Russia than I have been to Russia" creates an unbounded dependency by this lack of agreement since the subject "I" is a singular subject that cannot be divisible into comparatively "more," or transversely fewer, not without buying into an absurdity or extraordinary circumstance that part of the subject "I" can go to Russia while another part does not, like if the subject "I" were to amputate his or her arm and only that part of the subject "I" were to go to Russia.
By the way, someone saying this is probably trying to get across the point that they are not the only person who has been to Russia (e.g., I am not the only person who has been to Russia. More than just one person has been to Russia.).