Is it true that amount top up to CPF cannot be withdrawn?

To answer your question, Yes, its true, but I think you are not taking it from the right perspective.

Let's use an example

If we go with the current sum of 181k for FRS in 2020, and you do RSTU / OA transfer of say 100k, and the cumulative interest earned is 30k, then

OA balance at 55 = 15k
SA balance at 55 = 200k
Total reserved amount in SA = 100k + 30k = 130k
You elect FRS which is 181k.

To form the RA account, they will deduct the 130k reserved amount first, then deduct another 51k from SA to form the 181k. After the deductions,
Available remaining for withdrawal anytime:
OA account = 15k
SA account = 200k - 181k = 19k.

You will need to choose either the BRS / FRS / ERS when you turn 65-70 and elect when to start cpf life payout. It is an eventuality, and frankly, the cpf system is already primed that you cannot do further top-up or OA transfer once your SA balance hits the FRS amount, so you can NEVER EVER top-up or transfer more than the FRS amount.

I just think its not enough to be a concern, but I guess too many people freak out when they see this whole warning that you can't take it out.

edit the only scenario that I can think of a valid concern is when your reserved amount is more than FRS amount, but you want to elect only to go with BRS amount. in that scenario, I think you don't have a choice... You can only choose FRS or ERS but frankly I can't see why you would be worse off

Yes. RSTU monies are reserved fully for RA Account and eventually CPF Life. So you'll only get it as a form of annuity for life. Only Mandatory Contributed or Voluntary Contributed monies and its interest above FRS or BRS with property pledge can be withdrawn as a lumpsum from the balance in SA and OA after 55.​​​