This diagram is actually pretty simple. It looks worse than it is. All it uses are Lambdas (serverless functions), S3 buckets (object storage), and SNS (broadcast/push queues). There appears to be one traditional server in there with EFS, which is just an elastic file system.
All of these systems have equivalents in all the major cloud providers. So if the builder of this wanted to move to GCP or Azure, they are not really locked to AWS. This can all be built in another cloud.
Now, could you do it in a day? No. Assuming they are building it with Infrastructure as Code (such as Terraform) then they would need to convert the provider and change resource blocks. But this akin to refactoring in a codebase. Its work, but its not terribly difficult. Then they point it to their new cloud and run `terraform apply`.
There is almost no way to entirely remove vendor lock-in. The closest you could come is by designing everything yourself on bare metal servers and renting those from a cloud provider. So instead of using a managed queue system, you run some sort of messaging queue on the server. Then you host files on the server's filesystem, and you run the "lambdas" as applications on the server. But that almost causes more headaches than you save or solve for.
I look at Cloud Providers as similar to cell phone providers. I know people who live in fear of being locked into a contract with Verizon or something. But really, what are you going to do? You will always need a cell phone. The only other real choice is AT&T or maybe Sprint/TMobile. How often are you really going to switch and what are you really gaining by doing so? Energy spent worrying about being "locked in" to a cloud vendor is energy wasted. Yeah you can move from AWS to Azure or GCP. But that's about it. What do you gain by switching? Probably almost nothing. They are all pretty comparable at this point in reliability, features, and price (GCP is the slight laggard here, but not by much). If Google calls your company and offers you a huge discount to switch, you could still do it. Aside from that, there's minimal incentive to do so.
There are a few weird services that AWS has for example that might be considered "lock-in" services. This would be things like AWS Snowball or AWS Groundstation. These don't have comparable systems on other platforms. In the case of Snowball you probably have so much data on AWS that just transferring data would take months (or even years) which could be considered a form of lock-in.
tl;dr - This is a very tame arch diagram. A few lambdas, s3 buckets, and messaging queues, all of which have comparable services on all major clouds. There isn't significant vendor lock-in, this could be rebuilt fairly easily (assuming they used IaC) on any major cloud provider.