Abilities that let you do something new say "you can...". If, on the other hand, an ability is enhancing something you could already do, it will instead say "when you..."
Spoiler: nice summary
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As an example:
The first sentence is giving you the ability to do something you couldn't before (Help as a bonus action), so it uses "you can".
The second is improving something you could already do (Help a creature Attack), so it uses "when you"
Take another example:
Note the use of 'when you' at the start, because casting Mage Hand is something you can already do.
However, all the subsequent abilities are things you weren't previously able to do, and thus they use "you can".
Now look at the Assassin ability:
At no point does it say "when you..."
Thus, unless you are saying that WotC abandoned their writing style for this one ability, we must deduce that this entire ability is something only Assassins can do.
Otherwise it would say something to the effect of "When you use Disguise and/or Forgery kits to craft false personas and documents..." because it would then be something that other rogues can also do, and you merely get a better version of it.
Instead, the ability is clearly written in such a way as to indicate that crafting false personas is something you can only do if you are an Assassin of at least 9th level.