For Chromium builds which natively support Flatpak’s sub-sandboxing:
For Chromium builds which don’t and use zypak instead (i.e. Brave Browser) you will see “SUID” displayed, yes, but it is not Chromium’s old SUID sandbox even though it says it is:
Zypak works by using LD_PRELOAD to trick Chromium into thinking its SUID sandbox is present and still setuid, but all calls to it get instead redirected to another binary: Zypak’s own sandbox.
Additionally, even if it was using the official SUID sandbox, that sandbox is still maintained.