It depended on whether Anderson counts as a 20 year old. CapFriendly lists Anderson as 20 for CBA signing age. He doesn't turn 20 until June, but the CBA might go with birth year rather than birthdate. If Anderson counted as a 19 year old, then his deal would have slid.
ELC signing age is considered as the player's age as of September 15th of the calendar year he signs his ELC. Since Joey turns 20 in June, he's considered 20 for this ELC. (And would have been considered 19 if he signed before 2018, etc.).
Only 18 and 19 year old ELCs are eligible to slide so by simply signing his ELC to start this year, a year was burned even if Joey never even joined the team. If this was eligible to slide, the 10 game rule (as far as I know) still applies, so he would have to be in the lineup for 10 playoff games.. the CBA does not distinguish between NHL regular season games and playoff games.
If he played his junior year I think an ELC would be 2 years vs. staying 1 more year and being UFA eligible. Or am I wrong on that? I think the ELC thing is a little bit overblown. Yes, it's an advantage to have contributing ELCs, but smart RFA contracts can be a good substitute.
If he stayed 1 more year and then signed an ELC to begin immediately (i.e. signs similarly this time next year), his ELC signing age would be 21 and he still would have a 3 year ELC, with the first year burned immediately = 2 years effectively.
If he had stayed 2 more years (ELC signing age 22), he would then be eligible for a 2 year ELC. The nuance there would have been that had he signed immediately with the Devils, the first year could have been burned and it would effectively be a 1 year ELC. However, had he chosen to go to UFA, he would have to wait until the new league year and would not have been able to burn a year = 2 year ELC (see Butcher, Kerfoot).
I agree that there's very little that can hurt a team by burning ELC years. It's not a year accrued to FA, it's simply having to pay for an extra RFA year earlier on in the player's career.. which almost always will be cheaper than the potential 3.5M full cap hit of a max ELC. It looks like Joey's deal also has no performance bonuses so in this case, the team traded him a lower cap hit/salary for one year of his ELC.