For direct mounting, sure. But with a shorter FFD, Canon would provide an adapter for EF lenses, just as they do for the EF-M mount.
Of course, at issue is just how short they make that FFD. If they use a new FF mount that’s not EF-M, it’s manifestly in Canon’s best interest that those new ‘EF-X’ lenses can be mounted on EOS M bodies – in fact, I am sure Canon must ensure that possibility to provide an upgrade path for APS-C to FF analogous to DSLRs (EF mounts on APS-C). Custom EdMika-type adapters aren’t ergonomically feasible, so Canon likely needs to have at least ~12mm (like the EF12 extension tube) longer than the EF-M’s FFD for that adapter solution to work, meaning an FFD of >30mm. That means the ‘thin mount’ really only saves ~13mm from the current EF mount. I’d question whether that’s ‘worth it’. Also, that path means EOS M series users may need to carry two adapters, one for EF/EF-S to EF-M, and one for EF-X to EF-M.