I think this is industry-dependent.
Example: Google has a ton of engagement signals for restaurants. They generally get a ton of reviews. A ton of driving direction requests, calls, clicks to see the menu and maybe clicks to order online. People spend a lot of time on restaurant GBPs. Plus, Google can track foot traffic. I don't think a restaurant needs a strong website or GBP landing page to rank in the Local Pack (and anecdotally, a lot of the restaurants that rank in my area have lousy web sites).
OTOH, I work exclusively in real estate. (So take my restaurant comments above with a grain of salt.) Average real estate agent does ~10 sales per year, which means they get very few reviews. Most agents drive to their clients, so there's few driving direction requests and no foot traffic to track. There's not much in a real estate agent's GBP to engage with -- no menus, no order online, etc. And what I see in my work is that traditional SEO plays a pretty big role in Local Pack rankings -- both the landing page in the GBP, and the strength of the site overall.