Its definitely this. Having your identity linked to your messaging metadata is huge for state intelligence surveillance. Combine this with the fact that iOS and Google Play push notifications are sent their servers in plaintext, you then have a good setup to passively collect huge amounts of user data when they think they're being private. It doesn't help also that Signals database relies on the phones "secure" element for encryption. Placing trust in Apple's and Google's proprietary software to keep your messaging private.

These insecurities are addressed in the Molly client (android hard fork of the Signal client) on a FOSS ROM like GrapheneOS. It does not use Google push notifications and allows database encryption with a passphrase. Then using smspool.net you can get non KYC SMS codes paid with monero or lightning.

Even with all this though, nearly everyone else you communicate with will have the compromising defaults. This is what they want.