Yes, a vertical saas will have many of the same features. If they didn't have the feature, people would just buy the horizontal.
Slack does not come bundled with a spreadsheet and word processor though. MS Teams does - that's your horizontal. People who buy Slack generally don't buy Word.
It's a common strategy for a startup to go vertical, then horizontal. Slack was in the business of replacing IRC at first (hence the # logo), but kept going deeper.
HIPAA compliance is another vertical. It's not worth it for Slack to go after hospitals; it's a customer channel they don't understand or sell to. The apps that do hospital won't go after Slack's customers either. But both will go after the ones using MS Teams, email, or WhatsApp.
Another example is payment gateways. Stripe covers 46 countries. Yet nearly every country and region has their own localized payment gateway. Because Stripe doesn't have the depth to cover all the channels, every bank, every e-wallet. E-wallets are lighter than bank accounts, making them some of the most tightly regulated, especially for money laundering and countries with more crime/corruption. Stripe won't do these; most people with e-wallets have bank accounts and credit cards. It's a lot of work just to deal with one country, too much to cover 46 countries.