Only one way to find out! But I have to say that unless you'd said what it was I wouldn't have guessed it was a mango seed at all. What has happened to it?

That is an impossible question. You haven't said what happened to make it look like that. I doubt I've ever seen one looking like that and I've seen many Mango seeds over many years. But as Floral said, try it anyway.

I wouldn't waste the effort.

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Have you seen them do this? I haven't.

We normally would see little leaves on an unembedded stem.

Stem is not tender, and it is firmly embedded.
I am inclined to snip it at point of entry -- what do you think?

I'd try to ease it out as best as possible and leave what breaks off. There's a good chance that it will come good. I've seen a lot of "mutated" beginnings in mango seedlings that have grown out okay. But still keep a "Plan B" in reserve.

True, result may be stem slightly longer if it breaks than if clipped,
and probably no shorter at worst. Thanks.

Related :
As you see, I do not remove shells, thinking that they are probective or
otherwise beneficial.
Do you retain them, or remove them?

I never remove them, for much the same reason. If you try to remove the shell there's a good possibility you could damage the seed inside. Also I guess it's a lot less work involved by just leaving them.

In my previous photo, the MIDDLE of the stem was growing upwards and
outwards, because the tip could not proceed in chosen path
upwards through the heart of the seed,
for too much resistance in the interior of that one.

Next is another that took the same path.
Even though I expected this one to succeed, I was proactive :

That looks like one dead and rotting mango husk in the OP.