I don't think we can conclude that yet, but it is looking increasingly likely. One question remains though: if the Belgian royal family no longer owns the Spanish tiara, who owns it now ? Theoretically, it should belong to the "Help Funds of the Queen" if news about Queen Fabiola's will are to be believed. But, as far as I know, Queen Mathilde is the chair of the Help Funds and I doubt that charity organization would sell any of Queen Fabiola's jewels.
Another possibility, of course, is that Fabiola's Spanish family now has it, but I think we would probably know by now if that was indeed the case.
I still hope that Queen Mathilde is saving the tiara for a state visit to or from Spain, but my hopes are fading.
EDIT: A separate question is the fate of the stones that were worn with the tiara, especially the aquamarines from King Baudouin's gift and the Tunisian demi-parure. Are they gone too ? That includes not only the fleuron stones properly, but earrings, necklaces, pendants, etc. that would be of great use to Queen Mathilde. I am not an expert, but I suppose the aquamarines for example could even be reused if the Queen wanted to commission a new small tiara.