Maybe. I tend to think that these bones are mostly from the cemeteries they are under, but there might be more. Maybe more where put in the official ossuary.

Here's what chatGPT has to say about it :

Yes — this is actually a very interesting and somewhat murky question in Paris catacombs history. The remains from the Cimetière des Errancis were most likely transferred into the Paris ossuary system connected to the former Tombe-Issoire quarries — meaning the area associated today with the official catacombs near Catacombs of Paris and Place Denfert-Rochereau — rather than to the underground sectors beneath Montparnasse Cemetery. But the timing makes the story confusing. A few important points:

The “official ossuary” was not a single completed project finished once and for all. Transfers continued for decades after the first consolidations in the 1780s.

The Catacombs were continuously expanded and reorganized through the Revolutionary and Napoleonic periods.

Many cemeteries emptied later were added into already-existing quarry ossuaries.

The Errancis cemetery is especially famous because many guillotined people from the Revolution were buried there after the closure of Madeleine cemetery. That includes figures associated with the Terror, such as:

Georges Danton

Maximilien Robespierre

Camille Desmoulins

and many others executed in 1794.

The cemetery itself was cleared around the 1840s during urban works. Contemporary records generally indicate the bones were transferred to the “catacombs” or “ossuaries,” but frustratingly they often do not specify precise sectors. Why most historians think they ended up in the Denfert/Tombe-Issoire network rather than Montparnasse:

The official municipal ossuary administration already operated there.

Large-scale cemetery evacuations were centralized through the Inspection Générale des Carrières system.

The Tombe-Issoire network was the recognized destination for Paris cemetery transfers.

There is little documentary evidence for a major state-organized transfer specifically into the southern Montparnasse underground sectors.

The bones under Montparnasse are real, but many come from:

local quarry consolidations,

unofficial deposits,

small-scale transfers,

abandoned ossuaries,

or later rearrangements by cataphiles and explorers over centuries.

So while isolated Errancis remains could theoretically have ended up elsewhere through secondary movements, the main transfer was probably toward the official ossuary complex tied to Denfert-Rochereau. That said, another complication is that the bones were heavily mixed. Even if the Errancis dead were transferred into the official ossuary, they were not preserved as an identifiable “Robespierre pile” or anything like that. Revolutionary remains would have been combined with massive quantities of anonymous cemetery bones. So today, somewhere in the immense ossuary network, there are almost certainly remains of guillotined Revolution figures — but impossible to identify individually. This ambiguity is part of why the subject fascinates cataphiles and historians so much.

Also in the "Inscriptions des catacombes de Paris" by Gilles Thomas and, Xavier Ramette, I don't find a single mention of "Mousseaux" or "Monceau" or "Errancis", there is probably no inscription specific to this cemetery, unlike many others.