A workaround is to add a space between BEGIN and TRANSACTION, eg BEGIN TRANSACTION. Since having two spaces is a valid SQL command, it makes me think that it might be possible to bypass this restriction by adding an extra space to the BEGIN TRANSACTION command. For example, create a file elon.sql:

BEGIN  TRANSACTION;
INSERT INTO "X" ("id", "text") VALUES (314, "Formerly know as Twitter");
# 101k insert statements
COMMIT;

and run

npx wrangler d1 execute musk-db --file=elon.sql

NB: Transactions aren't currently supported yet, there is #2733 is implement this, however it is still open and in active discussion.

This, on an initial glance, seems to be caused by

The mayContainTransaction function checks for BEGIN TRANSACTION, but the trimSqlQuery attempts to remove statements BEGIN TRANSACTION; (with a semicolon) and COMMIT; (with a semicolon). As in your example, this will fail (and result in the multiple transaction error) when the string BEGIN TRANSACTION is found anywhere in a SQL statement, even when the string is representing a string value.

I have raised #8412 to identify a potential fix - I most likely won't have free capcity to write tests to test this out in the near future, so if anyone's got any spare time feel free to take this draft PR as inspiration.