It's what I think. The old hub 3s are breaking down with red light warning galore. They have older WiFi standards, and the manufacturer will not want to do firmware updates for security related vulnerabilities for ever.
VM are behind the curve compared to other ISPs who do high speed internet, so it makes sense to have a migration path, initially for the higher tiers, to reduce support costs and standardise and make speed upgrades for customers less of a pain. The HUB5 also makes use of Docsis3.1 channel which helps in over utilised areas and can bond the most downstream channels compared to other older hubs. I haven't checked, but it's probably more power efficient too and supports at least WiFi 6 AX technology which most new phones, tablets and laptops support now.
The pictures for broadband only on VMs web site also indicate a HUB5 for tiers down to M350. I know there is a disclaimer saying pictures are illustrative only, but if you buy via PERKSATWORK, it tells you in the VM checkout stage if you get a HUB5.