not sure why i remained stuck on this for a long time but you can call toInt() function on your strings to convert strings to int.
Reference:
https://developer.roku.com/docs/references/brightscript/interfaces/ifstringops.md#toint-as-integer
It's too big to be an int - BS has a 'long' but there's no "toLong" method.
Weirdly this works:
parseJson("1234567891000")
...that'll return a numeric value of type 'LongInteger'
[edit]Oh wow, only just noticed this is a thread dredged up from 2015, oops.