you got the whole process wrong.

RM1k/day limit is set at myptptn app and web portal. NOTHING to do with bank islam.
actual process:

1) Go to Bank Islam branch/ATM
2) Link your SSPN account at Bank Islam ATM
3) Once LINKED, you can use Bank Islam App/web portal to withdraw fund from SSPN account directly into Bank Islam SA only. RM0.50 transfer fee applies. Better to do this with large sum (iinm, up tp RM50k), rather than multiple small-ish amount.

... again, myptptn app/web is still not involved throughout the process...

4) Once SSPN fund credited into Bank Islam SA, just do the normal DuitNow Transfer from Bank Islam web/app to external banks. Transfer fee RM0.50 applies if >RM5k. Just make multiples of RM5k transfers, no fee applies.

Bank Islam allows me to do that.

Instantly

you both are talking 2 different method
what you did: use SSPN portal, perform withdrawal, selected Bank Islam as destination bank.

what MGM and other do: use BIMB portal, withdrawal from SSPN account into BIMB account directly.

But u have to open a B. Islam account.
Open a basic savings account with debit card n ZERO fee.