Short answer. Yes. Article 283 requires written notice on the workers and on the labour department at least one month before the intended date. Telling only the workforce does not satisfy the article; the filing with the Department is a separate requirement, not a courtesy.
What the law says
by serving a written notice on the workers and the Ministry of Labor and Employment at least one (1) month before the intended date thereof
Labor Code, Article 283 — Closure And Personnel Reduction. Read the full provision →
The article names two recipients
The employer's power to terminate for retrenchment, redundancy, labour-saving devices or closure is granted by serving a written notice on the workers and the Ministry of Labor and Employment at least one (1) month before the intended date thereof. The conjunction is and. The Ministry named in the text is today's Department of Labor and Employment, and the requirement did not lapse with the change of name. A retrenchment announced only inside the company has met half of what the article asks.
Why the second notice exists
Notice to the workers gives you a month to look for work. Notice to the Department does something the first cannot: it puts the reduction on record with an office outside the company, at a date the employer cannot later revise, and it lets the Department see the scale of the exercise while it is still prospective. That is why an internal memorandum, however clearly worded, is not a substitute. A record that exists only in the employer's own files can be produced, amended or lost by the party it burdens.
One month before the intended date
The period runs to the intended date of termination, so it has to be counted backwards from the day you actually stop working. Notice given three days before the last day is not cured by paying you for the remaining weeks, though what that shortfall entitles a worker to is a separate question from whether the retrenchment itself was justified. Keep the notice you received with its date, the envelope or transmittal if there was one, and any announcement to staff, because the timing is proved by documents rather than recollection.
How to find out whether it was filed
You are not left guessing. Ask the employer for a copy of the notice it served on the Department, with proof of filing. A company that complied has the receiving copy and no reason to withhold it; a company that did not will usually tell you as much by how it answers. If you intend to question the retrenchment, gather your notice, your payslips, your start date and the names of the others affected — whether the Department was told is one issue, and whether losses were genuinely being prevented is the other.