workout is just making that string to convert to proper variable.

Difference
Convert Date 60

Convert Date ${60}

https://robotframework.org/robotframework/latest/libraries/DateTime.html#Epoch%20time

In other words
Using --variable EPOCHDATE:60 leads to the same kind of variable as
using ${EPOCHDATE} 60 instead of ${60} in callWithArgument.robot?

Yes.
When not specified with in the braces ${} it is considered int as string.
you could check the same passing --variable EPOCHDATE:${60}

Ah ok. So I replaced the workaround by
${EPOCHDATE} Convert To Integer ${EPOCHDATE}

I also checked what happens with or without asserting the integer type when I run
robot --variable EPOCHDATE:${60} callWithArgument.robot
However the script fails with
'' cannot be converted to an integer: ValueError: invalid literal for int() with base 10: ''
Obviously the EPOCHDATE:${60} is resolved to EPOCHDATE:''
-> so I'll have to keep the type conversion line.

I'd suggest to add one further line as example:

How do you think about it?

Values you give from the command line are strings. Thus if you want a value you pass like --variable EPOCHDATE:60 to be considered an integer, it needs to be converted. Some keywords do conversion automatically, but Convert Date explicitly handles strings an numbers differently so you need to do conversion yourself.

Being able to pass non-string values from the command line would be sometimes handy. We already have #2946 about that so this issue can be closed.