Yes it's true. At Ably we support websockets, SSE and comet fallbacks (simple long-polling and streamed long-polling). It's less and less common but there are firewalls that fail to handle websockets correctly, or simply block them. I can't name specific companies/examples, but call centers are one example - the network and desktop environments are fully locked down.
We also see in these cases that streamed HTTP can also be broken by the firewall - for example a chunked response can be held back by the firewall and only forwarded to the client when the request ends, as a fixed-length response. Obviously that breaks SSE and means you can't just use streamed comet as a fallback when websockets don't work.
