I agree with the second paragraph, but that's only because the presumption is he'll be good, healthy, etc. for at least a substantial portion of the early contract. The Owner doesn't want to be the guy that finds out that Deshaun Watson, in fact, has regressed badly, and is replacement level, in year 2-3 of a 5 year fully gtd extension.
Lamar ain't signing the 10 year that Mahomes did. Very few ever do that. But a 5-6 year contract is very much in play. At least 4 years is pretty much the norm. Most do 5.
Let's say they do 5.
Years 1 & 2 will be guaranteed for all good QBs, so we can take them out of the picture - it's only Years 3, 4 and 5 where it's worth discussing the difference between fully guaranteed vs conventional QB contracts.
The worst case scenario is career ending injury for Lamar within the first 2 years. The Ravens would pay for 3 additional years of injury vs. a conventional QB contract.
For every QB, the current season is guaranteed, so a Y3 career-ender would mean 2 additional years of cost, a Y4 would mean 1 additional year and Y5 would be a wash because every QB is guaranteed.
So an early career ending injury, the nightmare scenario on a fully gtd deal, would result in a max. of 3 additional lean cap years where it's hard to compete- but it is quite an unlikely scenario.
That should be weighed against the far higher likelihood that the Ravens will struggle to compete if they lose their star QB and have to find a replacement. (And the draft picks we have to pour into that quest should be weighed against any trade compensation.)
The second worst result would be a severe, but not career-ending injury, that reduces Lamar's effectiveness.
In this case, the gap between a fully gtd deal and every other QB contract is narrower. Every team is going to keep hold of their star QB while he recovers, and they're going to give them every chance to play themselves back into form. Over that time, QBs with conventional contracts are going to trigger some of their current year guarantees anyway. That scenario is unlikely to leave the Ravens more than 2 years of salary worse off than without a fully Gtd deal.
The other risk is that Lamar's play declines and the Ravens want to cut him for performance reasons. That's a risk you take with all contracts but there are never enough QBs. There are always teams who believe they can revive the careers of struggling QBs (eg Wentz). The Ravens would be able to trade Lamar and get out from his contract fairly easily.
A 4 year deal would lessen all these risks but be less cap friendly.