This is what the Internal Revenue Manual says about the SoL on 5329.  They probably have something to back it up. 

4.71.27.1.6 (12-23-2020)
Overview of Form 5329 Examinations
This section provides procedures for examining and closing Form 5329.
Except for IRC 72(t) tax, Form 5329 taxes aren’t part of a discrepancy adjustment case.
Reminder: Don’t use Form 4549-E with a Form 5329 case.
Although often filed with a Form 1040, Form 5329 is a distinct return from the related Form 1040.
Form 5329 has its own statute of limitations date. The statute of limitations for tax required to be reported on Form 5329 begins to run when Form 5329 is filed.