All we're saying is that RSS is a dying tool -- it's simply not as effective in distributing content as it used to be. So rather than encouraging your blog readers to subscribe to your blog via RSS, encourage email subscription instead! The logic is simple: Email boosts traffic to your blog, since email subscribers get notified directly in their inbox when new content gets published. This is compared to RSS, which subscribers have to check manually.
They are wrong on both accounts. First RSS is not dying, only Google Reader is dying, but other RSS readers, or apps powered by RSS continue to have strong adoption.
Secondly, email is in no way a replacement for RSS. Yes, email can boost traffic to a site, but RSS can boost traffic as well. They are also wrong that users have to check RSS to get updates, while they don't for email. In both mediums, users need to check for updates whether its their inbox or a feed reader. In fact, both protocols are typically implemented using polling (or in some cases push updates) so there's really no difference.
Lastly, there's a clear distinction between feed readers and inboxes. Email is a two way communication channel, while RSS is a one way channel. Many people use email with a zero inbox strategy by deleting or archiving each item. On the other hand RSS is not optimized for that workflow but rather a browsing methodology where you read what you want and leave the rest.
From a product perspective, HubSpot is moving increasingly into email marketing. It's understandable that HubSpot wants to promote email as a channel, but it's disingenuous to suggest using email where it should not be used.