If I own a block of code, I can release it under as many licenses as I like, even if those licenses would totally conflict, and then each one of my users will need to track which license they received it under and adhere to those terms (only). So yes, I can give Users 1, 4, and 5 a copy under the GPL, and then users 2 and 3 a copy under some commercial license, user 6 under some GPL incompatible copyleft license, no problem.
The key is where you say "wishes to create another product/service that they don't wish to open source, but it's based on the GPL/SSPL codebase". You're not creating it based on the GPL codebase, you're creating it on the codebase you own. The fact you've previously licensed it under X, Y or Z licenses is irrelevant; you can always release it again, in whole or in part, with whatever license you like. (But of course, whatever you released under the GPL is still out there, under the GPL; there's no take backs!) And you can also, of course, use it yourself however you like.
The key is ownership. If you own the copyright on the code, you're fine. If you don't, you must adhere strictly to the license which the actual owners granted you. To the extent that MongoDB (the company) owns the copyright on all the code, they're free to use it how they like, and release it as often as they like under whatever licenses they like.