Actually, their report is much more credible because they note the difference between TorGuard (and a few similar apps) vs normal VPNs. To actually quote the report:
67% of Android VPN apps claim to provide traditional VPN services (labeled here as “VPN clients”) including enhanced security and privacy, anti-surveillance or tunnels to access geo-filtered or censored content. Note that we consider Tor clients (e.g., Orbot, Globus VPN] and TorGuard VPN client) as a separate category.
That seems a pretty fair distinction.
Notably, the report doesn't include the word "onion" in it at all, so I'm not sure where you got "[they] think it's related to the onion router" from.
Paper (PDF) at