Round and Turn are not synonyms.
From Player's Basic Rules, page 69, or the Player's Handbook, page 189 (bolding added for emphasis):
A round represents about 6 seconds in the game world. During a round, each participant in a battle takes a turn.
"On your turn" thus specifically excludes other characters' turns.
In the sections on Actions and Bonus Actions, it defines when each is allowed.
On your turn, you can move a distance up to your speed and take one action.
Various class features, spells, and other abilities let you take an additional action on your turn called a bonus action.
Then, in the section on the ready action (PBR 72, PHB 193)...
To be readied, a spell must
have a casting time of 1 action, and holding onto the
spell’s magic requires concentration (explained in
A casting time of "1 bonus action" is not the same as "1 Action". In fact, the general casting time is 1 action; a bonus action is specifically called out as different under "Casting Time" on PBR 79 and PHB 202...
A spell cast with a bonus action is especially swift. You
must use a bonus action on your turn to cast the spell,
provided that you haven’t already taken a bonus action
So, in looking at the letter of the rules, it's clear that you can't use a bonus action spell outside your turn, and further, it's explicit that your turn ends when someone else begins their turn.
Yes, you can use a bonus action then
You answered your own question :) That ruling on bonus action states that you choose when to use your bonus action unless that bonus action has other riders (like GWM allowing another attack if crit or reducing the enemy to 0 HP...meaning you can't take that bonus attack until that circumstance occurs.)
You are completely free to take your bonus action Hide at any point in your turn.
(However, you will still need to make the stealth check to hide - which may be at a penalty since the target may have seen you go to cover after striking.)
The rules on bonus actions (PHB, page 189) say:
This means that a tortle character can use a bonus action to emerge from their shell at the start of their turn, and proceed with the rest of their turn as normal.
As far as Rules as Intended are concerned, Crawford agrees.