If you made your statutory declaration under the grounds of 'made reps but did not receive a notice of rejection', then this regulation applies IMO:
Where a declaration has been served under paragraph (2)(b) or (c),[2(c) being 'made reps..' etc.] the charging authority shall refer the case to the adjudicator who may give such directions as he considers appropriate.https://www.legislation.gov.uk/uksi/2001/2313/regulation/19/madeIMO, you are not at adjudication, the authority has referred the matter to the adjudicator for 'directions'. These are likely to be that you should provide evidence to the adjudicator that you submitted reps(because the authority's evidence says that you did not). If you convinced them-by submission of written evidence- then the matter would be registered as an appeal. If you did not, then the adjudicator would likely direct you to pay the full penalty without an appeal being heard.
IMO, there's no option to be issued with a fresh PCN because your grounds were not related.
TfL's response to you is misleading as regards what happens, but this isn't a defence. Your next communication should be from the tribunal and, as posted by stamfordman, I suggest you contact them (the tribunal) to find out the current position